xhci: remove conversion from generic to pci device in xhci_mem.c

This patch removes the to_pci_dev() conversion performed to generic struct
device since it is not actually useful (the pointer to the generic device
can be used directly rather through a conversion to pci_dev) and it is pci
bus specific.

This isn't stable material because this code will produce harmless
behavior on non-PCI xHCI hosts.  The pci_device pointer is never
dereferenced, only used to re-calculate the underlying device pointer.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>