misc: pci_endpoint_test: Check SUCCESS bit for doorbell status

The pci-epf driver sets STATUS_DOORBELL_ENABLE_SUCCESS as the final step of
pci_epf_test_enable_doorbell(), and STATUS_DOORBELL_DISABLE_SUCCESS as the
final step of pci_epf_test_disable_doorbell(). A missing SUCCESS bit
therefore unambiguously means that the operation did not complete, whereas
the FAIL bit is only set on an explicit failure path.

The host side test in pci_endpoint_test_doorbell() currently keys off
the FAIL bit. That covers explicit failures but misses two cases.

The first case is when the wait for the completion IRQ times out. No IRQ
arrives, the Endpoint never updates STATUS, and neither SUCCESS nor FAIL
is set. The enable path already handles this correctly because it also
fails when the wait times out without an IRQ. The disable path does not
have that extra guard and would wrongly treat the timeout as success.

The second is a buggy EPC that raises two IRQs in response to a single
DOORBELL_ENABLE command. The second wait_for_completion_timeout()
returns immediately with 'left' non zero, but the endpoint has not yet
written STATUS, so SUCCESS is clear and FAIL is also clear. The current
FAIL only check treats this as success.

So check the SUCCESS bit instead. That matches the Endpoint's contract
because SUCCESS is the last write on the success path, and it correctly
reports failure for both timeouts and the spurious IRQ case without
relying on the FAIL bit being set.

Fixes: eefb83790a0d ("misc: pci_endpoint_test: Add doorbell test case")
Signed-off-by: Niklas Cassel <cassel@kernel.org>
[mani: commit log]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Link: https://patch.msgid.link/20260730122045.1382749-5-cassel@kernel.org

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