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

This reverts commit ff25c3b17479ae7101074f8b887e890eaa68a94e.

Move back the allocation of netdevice to alloc_inst(), one-time
registration to bind(), deregistration and free to rm_inst(). The
UI update issue will be taken up with proper stakeholders.

Change-Id: I56448b08f6796a43ec5b0dfe0dd2d42cdc0eec14
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>