V4L/DVB (7218): Fix breakage in mt9m001 and mt9v022 driver if "CONFIG_GENERIC_GPIO is not set"

Both camera drivers can function without GPIO support, in which case they
will only support the 10 bit data width mode. But the two respective switch
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>