[media] v4l: Fix ARGB32 fourcc value in the documentation

The ARGB32 pixel format's fourcc value is defined to 'BA24' in the
videodev2.h header, but documented as 'AX24'. Fix the documentation.

Reported-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>