PCI/ASPM: Disable/restore ASPM on every function for multi-function devices

pcie_aspm_cap_init() disables ASPM L0s/L1 before touching L1SS config, then
restores the pre-existing state afterward. Both steps only ever touched
link->downstream, i.e. function 0 of the downstream component, leaving
sibling functions (>0) on a multi-function device untouched.

This means the "disable" step does not actually disable ASPM link-wide on a
multi-function device: a sibling function can still have L1 enabled even
after this step runs. PCIe r7.0, sec 7.5.3.7, recommends programming the
same ASPM Control value for all functions of a multi-function device, and
pcie_config_aspm_link() already loops over every function on the bus for
exactly this reason.

Loop over every function on linkbus->devices for both the disable and
restore steps, keeping the existing sec 7.5.3.7 ordering (disable
downstream functions before upstream, restore upstream before downstream
functions). The masked pcie_capability_clear_and_set_word() accessor from
the previous commit makes this safe: it only ever touches the ASPM Control
bits, so function-specific bits elsewhere in LNKCTL (e.g. Read Completion
Boundary, CLKREQ Enable) on sibling functions are left untouched.

Fixes: 7447990137bf ("PCI/ASPM: Disable L1 before disabling L1 PM Substates")
Closes: https://lore.kernel.org/all/20260721143945.86E7D1F000E9@smtp.kernel.org/
Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20260727-aspm-v6-2-2ebb3ee7ef71@oss.qualcomm.com

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