[media] ivtv-alsa: Add index to specify device number

When using multiple capture cards, it might be necessary to identify a
specific device with an ALSA one. If not, the order of the ALSA devices
might have no relation to the id of the radio or video device.

Signed-off-by: Nicolas Sugino <nsugino@3way.com.ar>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>