[media] smiapp: Check for GPIO validity using gpio_is_valid()

Do not use our special value, SMIAPP_NO_XSHUTDOWN.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>