V4L/DVB (7217): Replace NO_GPIO with gpio_is_valid()

Upon suggestion by David Brownell use a gpio_is_valid() predicate
instead of an explicit NO_GPIO macro. The respective patch to
include/asm-generic/gpio.h has been accepted upstream.

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