have to depend on CONFIG_GENERIC_GPIO. Additionally remove redundant
gpio_is_valid tests - they are repeated in bus_switch_request() functions.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>