HTC header carries junk values that may be interpreted by the firmware
differently. Enable credit update only if flow control is enabled for
the corresponding endpoint.

Signed-off-by: Alagu Sankar <alagusankar@silex-india.com>
Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>