usb: gadget: ether: Move probe function above driver structure

Move the driver probe function above the driver structure, so it
can be placed alongside other related functions, like upcoming
remove function. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Tom Rini <trini@konsulko.com>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>