Use gpio_is_valid() to replace "gpio >= 0" or "gpio < 0".

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Hui Wang <jason77.wang@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>