[media] ivtv-alsa: Remove EXPERIMENTAL from Kconfig and revise Kconfig help text

Remove the (somewhat meaningless?) dependency on EXPERIMENTAL for the
ivtv-alsa driver.  Revise the Kconfig help text to be a little clearer for
the lay person, while we are here.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>