ANDROID: GKI: usb: 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.

Test: build
Bug: 153446435
Change-Id: I312772367a2cd293982a66ea8b75e04a8b1f2fd0
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
(cherry picked from commit 106c645a79ec0ba4695b1c2ca6055664cdbaa750)
[hridya: resolved minor conflicts, EXPORT_SYMBOL -> EXPORT_SYMBOL_GPL,
added some null pointer checks]
Signed-off-by: Hridya Valsaraju <hridya@google.com>