ASoC: fsl_ssi: Use the tolower() function

Code can be simplified by using the standard tolower() funtion.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>