sunxi: Fix USB_GADGET implication

USB_GADGET will fail to compile if USB_MUSB_GADGET is not defined. Make
sure we have that condition right.

Fixes: e0ea88042d51 ("sunxi: Imply USB_ETHER")
Suggested-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>