ALSA: pcm_dmaengine: Remove hardcoded PCM formats

Use the standard PCM helper function to figure out the sample bytes
instead of hardcodec PCM format checks in
snd_hwparams_to_dma_slave_config().

The patch also extends the format check for 8 bytes formats although
no one should match so far.

Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>