misc: pci_endpoint_test: Do not write status in IRQ handler

misc: pci_endpoint_test: Do not write status in IRQ handler

pci_endpoint_test_irqhandler() always rewrites the status register when an
IRQ is raised, either as-is if STATUS_IRQ_RAISED is not set, or with
STATUS_IRQ_RAISED cleared if that flag is set. The first case creates a
race window with the endpoint side, meaning that the host side test driver
may end up reading what it just wrote, thus losing the real status as set
by the endpoint side before raising the next interrupt.  This can prevent
detecting that the STATUS_IRQ_RAISED flag was set by the endpoint.

Remove this race window by not clearing the STATUS_IRQ_RAISED status flag
and not rewriting that register for every IRQ received.

Link: https://lore.kernel.org/r/20230415023542.77601-17-dlemoal@kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>