ASoC: SOF: Intel: Rename deprecated DMIC IPC struct field

ASoC: SOF: Intel: Rename deprecated DMIC IPC struct field

This patch restores the field name to fifo_bits_b since the
legacy firmware compatibility code (for firmware ABI 3.0.0 or earlier)
sets it in sof_link_dmic_load() function in topology.c. Setting of
reserved_2 didn't look appropriate.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Link: https://lore.kernel.org/r/20200415202816.934-19-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>