xhci: Restore event ring dequeue pointer on resume.

The xhci_save_registers() function saved the event ring dequeue pointer
in the s3 register structure, but xhci_restore_registers() never
restored it.  No other code in the xHCI successful resume path would
ever restore it either.  Fix that.

This should be backported to kernels as old as 2.6.37, that contain the
commit 5535b1d5 "USB: xHCI: PCI power
management implementation".

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Tested-by: Elric Fu <elricfu1@gmail.com>
Cc: Andiry Xu <andiry.xu@amd.com>
Cc: stable@vger.kernel.org