usb: u_bam: don't access f_rmnet

f_rmnet struct is accessed via grmnet struct to get the gadget
device handle. Functions other than rmnet like MBIM can also
use the u_bam module. Add the gadget struct in grmnet struct.

Change-Id: I45d05db87d01f402c7aaab06be926d21ee5241a0
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>