ASoC: qcom: audioreach: support WSA speakers only on WSA2

Neil Armstrong <neil.armstrong@linaro.org> says:

The Qualcom SM8650 based Ayaneo Pocket S2 gaming device has a set
of 2 WSA speakers connected on the WSA2 lines.

But the Audioreach DSP only handles WSA2 in pair with the WSA
interface by using the upper bits of the active_channels_mask
for WSA2 and the lower bits for WSA:

 __________________________________________________
| Bits  |     3    |     2    |   1     |     0   |
---------------------------------------------------
| Line  | WSA2 Ch2 | WSA2 Ch1 | WSA Ch2 | WSA Ch1 |
---------------------------------------------------

Setting only the WSA2 upper bits is perfectly valid and
functional but the current Audioreach code builds the bitmask
from the channels count with:
	active_channels_mask = (1 << num_channels) - 1;

In order to enable the WSA2 bits the channel count should be 4,
but the lower WSA bits are then also enabled and the DSP errors
out when trying to play on the disabled WSA interface.

A solution would've been to add a fake WSA2 topology element which
would be translated into the top bits only, but it's not clean and
add some special exceptions in the generic Audioreach code.

The solution suggested by Srinivas is to use the channel mapping to
set this bitmask.

This works but makes all the other calls using the channel mapping fail
because the DSP requires the channel_mapping table to start from index 0
and using num_channel length in order to apply the mapping on the
active_channels_mask bits in order.

So if you have an active_channels_mask like:
 _______________________
| Bits  | 3 | 2 | 1 | 0 |
-------------------------
| Mask  | 1 | 1 | 0 | 0 |
------------------------

And the channel mapping:
 _________________________________
| Channel |  3 |  2 |   1  |   0  |
-----------------------------------
| Mapping | FR | FL | NULL | NULL |
-----------------------------------

Will need to be converted to:
 ____________________
| Channel |  1 |  0 |
---------------------
| Mapping | FR | FL |
---------------------

And the DSP will map FR on WSA2 Ch2 and FL on WSA2 Ch1 as:
 _________________________________
| Line    |  WSA2 Ch2 | WSA2 Ch1 |
----------------------------------
| Mapping |     FR    |     FL   |
----------------------------------

So we need to skip the empty channel mapping entries in all other
users of the channel_map to build valid channel_mapping tables.

This should not break any other usecases since the default channel
mapping always start from index 0, and will add flexibilty to allow
some special non linear mapping for other interfaces as well.

Finally we add a special prepare callback for the Ayaneo PS2 sound card
which sets the custom channel mapping for the WSA interface to make audio
work properly on the gaming device.

Link: https://patch.msgid.link/20260728-topic-sm8650-ayaneo-pocket-s2-wsa2-fix-v3-0-b29f44720178@linaro.org

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