staging: gpio-mt7621: Include the right header

GPIO drivers should include <linux/gpio/driver.h> only, the
<linux/gpio.h> header is deprecated.

Cc: John Crispin <john@phrozen.org>
Cc: Zhiyong Tao <zhiyong.tao@mediatek.com>
Cc: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>