media: em28xx: fix input name for Terratec AV 350

Instead of using a register value, use an AMUX name, as otherwise
VIDIOC_G_AUDIO would fail.

Cc: stable@vger.kernel.org
Fixes: 766ed64d ("V4L/DVB (11827): Add support for Terratec Grabster AV350")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>