ASoC: doc: Replace open code with params_set_format()

The example code in dpcm.rst contains an open code calling
snd_mask_set(), and this can be better represented with
params_set_format() instead.  This automatically fixes the sparse
warning about snd_pcm_format_t handling, too.

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