[ALSA] ymfpci: add S/PDIF-in > S/PDIF-out loop

Modules: YMFPCI driver

This patch adds a new mixer control called 'IEC958 Loop' which makes
it possible to loop digital signals from S/PDIF-in to S/PDIF-out.

Signed-off-by: Glen Masgai <mimosius@gmx.de>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>