usb: gadget: u_bam: do runtime_put even if BAM channel was not opened

gbam_disconnect_work returns without doing a put, in the case where
bam channel was not opened successfully. This leads to a runtime
mismatch and USB not entering LPM again. Fix this by making sure that
we do runtime_put in above mentioned case.

Change-Id: Ic05f0ddf7ede3ad680726b9360a5d26ebf35772a
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>