usb: dwc3: Handle suspicious GEVNTCOUNT

In rare cases it is observed that dwc3_check_event_buf() will
read a suspiciously high GEVNTCOUNT (0xFFFC) which is obviously
larger than the configured event buffer itself. This seems to
come up even after the GEVNTCOUNT register was written as 0 in
dwc3_event_buffers_setup(). Work around this issue by ignoring
the incorrect high count and treating it as no events.

Change-Id: Id0d7e3ec85ed043c58dff9c529fd3bec9cd2b8b0
Signed-off-by: Jack Pham <jackp@codeaurora.org>