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

Driver registers net device in bind but does not unregister it on
driver unbind. Upon composition switch rndis 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: I19f26745e4e1477a71f8dd70cdb0b201c82e77b7
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>