usb: gadget: Fix response available notify count usage

The outstanding responses are purged and notify count is reset
during bus suspend. But the notify interrupt endpoint is not flushed.
If a request is pending with the controller, It gets finished during
resume and the notify count is decremented in the completion handler.
As the notify count is reset during the suspend its become negative now.
As the counter is not zero, interrupt notifications are queued even
though there are no responses. This is leading to watchdog bark due to
an error print messages.

Flush the notify endpoint before resetting the notify counter during
suspend. As we are modifying notify counter under spin lock, atomic
variable is not required. Ensure that notify counter never
becomes negative.

CRs-Fixed: 607029
Change-Id: I78cb7a585c1c2b527ca6b46bc99e7db8067ee21a
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>