ASoC: tas571x: remove improper PDN signal usage in set_bias_level

ASoC: tas571x: remove improper PDN signal usage in set_bias_level

The set_bias_level toggles the PDN signal when entering
SND_SOC_BIAS_STANDBY and SND_SOC_BIAS_OFF. However this has no effect and
actually breaks things down (tested with TAS5717) due to the following
reasons:

1) holding down PDN does not save power but holding down RST does
2) now hard mute via register 0x5 is implemented and therefore it is no
   longer needed to toggle PDN to enter all channel shut down
3) in order to leave PDN it is required to toggle the RST signal (see
   TAS5721 datasheet), which was not implemented
4) toggling PDN as implemented actually mutes PWMs and there is no audio
   output (tested on TAS5717)

For these reasons remove the PDN signal toggling and just initialize it to
inactive in probe().

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Signed-off-by: Mark Brown <broonie@kernel.org>