usb: xhci: Acknowledge pending events in secondary event ring

As part of bus suspend if xHC is halted when there are pending
events in the event ring, xHC stops responding to xhci commands
upon host or device initiated bus resume. Fix this issue by
going over all the pending events and writing address of xHC
event ring enqueue pointer to ERDP register. This indicates to
xHC that system software has processed all Events in the ring.

Change-Id: I0c1fafb03c9848d043599fcee19e9bf07c3a3acd
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>