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>

board/sunxi/board.c

@@ -733,7 +733,9 @@ int misc_init_r(void)
 		return ret;
 #endif
 
+#ifdef CONFIG_USB_ETHER
 	usb_ether_init();
+#endif
 
 	return 0;
 }