xen/pcifront: Drop pcifront_common_process() tests of pcidev, pdrv

xen/pcifront: Drop pcifront_common_process() tests of pcidev, pdrv

pcifront_common_process() exits early if pcidev or pcidev->driver are NULL,
so simplify it by not checking them again.

[bhelgaas: split flag change to separate patch]
Link: https://lore.kernel.org/r/20211004125935.2300113-4-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>