usb: gadget: move the global the_dev variable to their users

the u_ether.c file has a global variable named the_dev which keeps a
pointer to the network device after it has been created via
gether_setup_name(). It is only used internally by u_ether. This patches
moves the variable to its users and passes it via the port.ioport where
it is saved later anyway.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>