PCI/switchtec: Fix a MRPC error status handling issue

PCI/switchtec: Fix a MRPC error status handling issue

If an error is encountered when executing a MRPC command, the firmware
will set the status register to SWITCHTEC_MRPC_STATUS_ERROR and return
the error code in the return value register.

Add handling of SWITCHTEC_MRPC_STATUS_ERROR on status register when
completing a MRPC command.

Link: https://lore.kernel.org/r/20211014141859.11444-3-kelvin.cao@microchip.com
Signed-off-by: Kelvin Cao <kelvin.cao@microchip.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>