ASoC: fsl: fsl_ssi: fix kernel-doc

Fix W=1 warnings. The kernel-doc support is partial, add more
descriptions and follow proper syntax

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/20200702192141.168018-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>