usb: smd: Fix crash during reset when RmNet is connected

When a QMI/RMNET control connection is established, a diag
reset causes a crash due to a NULL pointer dereference. The
SMD channel pointer is set to NULL upon USB cable disconnect,
but a queued work function might still try to access that pointer.

Fix the crash by checking for NULL pointers in both smd_* and in
USB gadget transport functions that could pass a closed handle.

CRs-fixed: 336850
Change-Id: I87cb109f3e7007efe15b5acc81180151dd2ef023
Signed-off-by: Jack Pham <jackp@codeaurora.org>