usb_ether: register usb ethernet gadget at each eth init

usb_ether: register usb ethernet gadget at each eth init

Since the ether may not be the only one usb gadget would be used
in the uboot, it is neccessary to do the register each time the
eth begin to work to make usb gadget driver less confussed when
we want to use two different usb gadget at the same time.

Usb gadget driver could simple ignore the register operation, if
it find the driver has been registered already.

Signed-off-by: Lei Wen <leiwen@marvell.com>