ASoC: tas2552: Simplify and reverse the functionality of tas2552_sw_shutdown

The function name and parameters of:
tas2552_sw_shutdown(struct tas2552_data *tas_data, int sw_shutdown)

implies that if sw_shutdown is 1 we should be entering to the software
shutdown mode.

The code can be simplified as well within the function.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>