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

This reverts commit b26b39948b03892071104c91cddb479e54fd215c.

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: I844ecda80197d7bb83860473723c431d52686b1f
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>