Mixer: Check validity of input value

When the user wants to set a mixer with a value greater
than the maximum or smaller than the minimum, the mixer
is set at an undefined value.

This patch set purpose is to return a error if the input
value is out of range.

Signed-off-by: Baptiste Robert <baptistex.robert@intel.com>
Signed-off-by: David Wagner <david.wagner@intel.com>
Author-Tracking-BZ: 129089
Merge pull request #40 from dawagner/check-mixer-input-value

Mixer: Check validity of input value