ASoC: dmaengine: Use standard pcm_format_to_bits() macro

The conversion from PCM format type to bits needs an explicit cast,
and it'll be uglier.  Since we have a standard macro for that, let's
use it instead.

This patch fixes the sparse warning:
  sound/soc/soc-generic-dmaengine-pcm.c:200:63: warning: restricted snd_pcm_format_t degrades to integer

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>