usb: gadget: Start write_tobam work only from USB bus resume

This fix solves an issue where start_rx() and write_tobam()
create a recursive infinite loop causing a 100% CPU utilization.
The proposed solution is to queue write_tobam() work only when
start_rx is called as part of the resume sequence.

Change-Id: I4ff664bd6e8afc66fa5dec169df443b624c86995
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>