ASoC: soc-pcm: fix sig_bits determination in soc_pcm_apply_msb()

In the SNDRV_PCM_STREAM_CAPTURE branch in soc_pcm_apply_msb(), look at
sig_bits of the capture stream, not the playback one.

Spotted by coverity.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org