media: v4l2-device.h: add v4l2_device_supports_requests() helper

Add a simple helper function that tests if the driver supports
the request API.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>