usb: gadget: ether: Use plain udevice for UDC controller interaction

Convert to plain udevice interaction with UDC controller
device, avoid the use of UDC uclass dev_array .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>