ASoC: fsl_sai: Update is_slave_mode with correct value

is_slave_mode defaults to false because sai structure
that contains it is kzalloc'ed.

Anyhow, if we decide to set the following configuration
SAI slave -> SAI master, is_slave_mode will remain set on true
although SAI being master it should be set to false.

Fix this by updating is_slave_mode for each call of
fsl_sai_set_dai_fmt.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>