usb: gadget: f_rmnet: Fix binding of ETHER transport

gether_connect() requires gether->ioport to be set, otherwise
the net device won't be created. Set this pointer during
the f_rmnet bind call and clean it up in unbind.

Change-Id: I633a11e12e0f5c97d3c5cf7f4f0b517387ee373c
Signed-off-by: Jack Pham <jackp@codeaurora.org>