usb: gadget: Add NULL pointer checking in bam_data_resume_work()

Add NULL pointer checking to beginning of the bam_data_resume_work()
function in order to avoid potential crashes when a suspend or a disconnect
event has happened in prior to the execution of the resume work.

Change-Id: I170d404185de700cbdfe068ff4dbd833ea4d745e
Signed-off-by: Danny Segal <dsegal@codeaurora.org>