usb: gadget: f_ncm: allocate/free net device upon driver bind/unbind

Driver registers net device in bind but does not unregister it upon
driver unbind. Upon composition switch ncm net device is no longer
in use, hence unregister and free it in driver unbind. Unregistering
net device sends notification to user space which can be used by user
space entities to perform necessary actions for example updating UI.
Symmetrically allocate and register net device in driver bind.

Change-Id: Ie1bb781aba8efee20cc98c1d6bf264403c3b087e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>