ASoC: fsl_ssi: Remove comment about SSI running only in slave mode

Current driver can also run in I2S master mode, so remove the old comment.

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