drivers: usb: gadget: fix crash in u_bam_dmux

While usb compostion changes between rmnet and mbim, crash observed
during debugfs and bam cleanup operations.

Change-Id: I9ebfe9a2d0f556f4aa430ef7cc76a2e73f18aecd
Signed-off-by: Prasadarao Durvasula <pdurvasu@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>