usb: gadget: Fix NULL pointer access if NCM not instantiated

ncm_ctrlrequest() will be called in android_setup(), in the case
NCM instance is not created, structure _ncm_setup_desc is not
created, it will run into a NULL pointer exception.

Fix this by checking _ncm_setup_desc is NULL or not.

Change-Id: If8048a03a7ab46138739472327a17c65ce158382
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>