usb: gadget: u_bam: Fix race of finish_resume with disconnect

Add protection to bail out from gbam_start called by
usb_bam_finish_resume() in case usb cable was disconnected
by the time resume work was queued.

CRs-Fixed: 691643
Change-Id: I74318d3d72ce37dbe7403917366fbee2703b9ac6
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>