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

This reverts commit d44dbd5aa7f6bbb5008092564223d47c0b1e5fac.

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: Ifbd91b3cbb045851e81d1d7c64479f74120f0b24
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>