sunxi: only init USB Ethernet gadget when it's enabled

If the USB Ethernet gadget is not yet enabled, the call of
usb_ether_init in board/sunxi/board.c will lead to undefined reference
error when building.

Fix this problem.

Fixes: 50ddbf1199a0 ("sunxi: Register usb_ether")
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>