ASoC: Intel: Add FE rate & channel constraints for bxt-rt298

We support stereo 48Khz audio, so add these as constraints
for this card

Signed-off-by: Senthilnathan Veppur <senthilnathanx.veppur@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>