ASoC: fsl_ssi: No need call of_device_is_available()

The comment and the call to of_device_is_available() are not really
needed.

It is the expected behaviour to probe only the ssi nodes that are
enabled in the device tree.

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