usb: gadget: u_bam: Clean up gbam_connect and gbam2bam_connect_work

Fix NULL pointer dereference issues in gbam_connect() and
gbam2bam_connect_work(). Also clean up the code to remove
redundant checks.

Change-Id: I03f7f1224f89a52d11c6062cea1835ca38276b52
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>