[media] v4l2-controls.h: fix copy-and-paste error in comment

The comment for the FM_RX class was copied from the DV class unchanged.
Fixed.
Also made the FM_TX comment consistent with the others.

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