staging/easycap: Add option to set the hardware audio gain

A new module parameter adjusts the gain of the AC'97 audio chip, if
one is present.  Attenuation as well as amplification should be possible
according to the datasheet, but attenuation seems not to work yet.

Signed-off-by: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>