ASoC: Intel: Skylake: Add 16-bit constraint to FE bxt_rt298 machine

Add constraint to FE to restrict sample format to 16-bit for bxt_rt298
machine

Signed-off-by: G Kranthi <gudishax.kranthikumar@intel.com>
Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>