usb: gadget: Fix connect/disconnect/suspend/resume race conditions

This patch add synchronization protection between connect, disconnect,
suspend and resume events and their corresponding works in the u_bam and
in the u_bam_data_drivers. In addition, better manage the work-queue and
ignore works when needed (for example, ignore everything if last event
was disconnect).

CRs-Fixed: 678217
Change-Id: I64b45184183495f9ee37bc1c820dc83306e777f5
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>