usb: xhci: Add helper API to issue stop endpoint command

This API is used to issue stop endpoint command on
requested endpoint in order to retire all active TRBs
in the transfer ring.

Change-Id: I312772367a2cd293982a66ea8b75e04a8b1f2fd0
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>