ASoC: cht_bsw_rt5672: Use snd_pcm_hw_constraint_single()

Use the new snd_pcm_hw_constraint_single() helper function rather than
installing a list constraint with a single value. Since
snd_pcm_hw_constraint_single() sets a static constraint while
snd_pcm_hw_constraint_list() sets a dynamic constraint the former is
slightly more efficient and it also needs less code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>