[media] em28xx: add support for g_chip_name

Add support for vidioc_g_chip_name, allowing AC97 to be implemented as a
second chip on the bridge with the name "ac97". v4l2-dbg can just match the
name with that string in order to detect a ac97-compliant set of registers.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>