drivers: usb: gadget: Change gbam setup usage in rmnet function

rmnet and mbim functions both use same bam channel 8 for bam setup.
gbam setup is called when rmnet function get installed during usb
composition. Change gam setup usage to bind_config call during usb
config. gbam cleanup is also changed to unbind call in rmnet.

Change-Id: I654ff0e127864e549568032bd38d1ce9d5e28543
Signed-off-by: Prasadarao Durvasula <pdurvasu@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>