vfio/pds: Use proper PF device access helper

vfio/pds: Use proper PF device access helper

The pci_physfn() helper exists to support cases where the physfn
field may not be compiled into the pci_dev structure. We've
declared this driver dependent on PCI_IOV to avoid this problem,
but regardless we should follow the precedent not to access this
field directly.

Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230914021332.1929155-1-oushixiong@kylinos.cn
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>