ASoC: fsl_ssi: Clean up helper functions of trigger()

The trigger() calls fsl_ssi_tx_config() and fsl_ssi_rx_config(),
and both of them jump to fsl_ssi_config(). And fsl_ssi_config()
later calls another fsl_ssi_rxtx_config().

However, the whole routine, especially fsl_ssi_config() function,
is too complicated because of the folowing reasons:
1) It has to handle the concern of the opposite stream.
2) It has to handle cases of offline configurations support.
3) It has to handle enable and disable operations while they're
   mostly different.

Since the enable and disable routines have more differences than
TX and RX rountines, this patch simplifies these helper functions
with the following changes:
- Changing to two helper functions of enable and disable instead
  of TX and RX.
- Removing fsl_ssi_rxtx_config() by separately integrating it to
  two newly introduced enable & disable functions.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Caleb Crome <caleb@crome.org>
Tested-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>