USB: u_bam: Replace usage of port->gr with port_usb

port->gr and port_usb serves same purpose and hold address
of grmnet. Clean it up by removing duplicate 'gr' field.

Change-Id: I89ade95ced80076d84e84ba39f579b69af1a243e
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>