usb: gadget: udc: Drop surplus include

The UDC NET2272 driver includes <linux/gpio.h> but does not
use any symbols from this file, so drop the include.

Cc: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>