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, leading to potential
use-after-free.

Fix this by adding check gadget function bind or not.

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