[media] em28xx: fix audio input for TV mode of device Terratec Cinergy 250

[media] em28xx: fix audio input for TV mode of device Terratec Cinergy 250

Remy Blank reported that audio over USB can be made working for the television
input if .amux is changed from EM28XX_AMUX_LINE_IN to EM28XX_AMUX_VIDEO.
An examination of his devices shows, that it is indeed supplied with an EM202
AC97 audio IC. We also use this setting for the Cinergy 200.
Remy Blank also provided the original version of this patch (many thanks !).
Fixes bug 14126 (see bug report for further device details).

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Remy Blank <remy.blank@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>