USB: u_bam: Handle error path properly in gbam_connect()

when USB is connected, composite layer calls each function driver's
set_alt() function. As part of that, RMNET driver tries to enable
USB IN and USB OUT endpoints. If usb endpoint enable fails, it is just
returning without doing proper cleanup. This might lead to memory leak
or crashes later. Fix the issue by handling error path correctly.

Change-Id: If0b8b6ce4ed4a638f704405d3adac64bee74d87c
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>