usb: gadget: Add check gadget function bind or not

For NCM gadget function, net device structure opts->net
will be freed when NCM unbind, but user space related
device node doesn't know it, will access net structre
without checking it exist or not, finally lead to crash.

Fix this by adding check gadget function bind or not.

Change-Id: I6af9e3868f71e7fa923600721e1f716508de9b55
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>