usb: xhci: Clear event handler busy flag upon event ring cleanup

There is a possibility of unacknowledged pending interrupt at the time
of secondary event ring cleanup. As a result xHC does not generate any
interrupt further because event handler busy flag is set. Hence disable
interrupter to prevent any further interrupt, acknowledge any pending
interrupt and clear event handler busy flag. This allows xHC to generate
interrupt after setting up new event ring.

Change-Id: I1b1791eacbf7afffd101d56d0ae06fe237b8c076
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>