Fix wrong setting on number of channels.  The context wants to set
constraint to 2 channels instead of 4.
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>