ASoC: tas571x: implemented digital mute

The driver did not have a mute function. The amplifier was brought out of
shutdown mode (hard-mute) once for ever in probe(), which was causing
clicks and pops when altering the I2C register configuration later.

This adds the digital_mute() function. The amplifier unmute in probe()
was removed.

Signed-off-by: Petr Kulhavy <petr@barix.com>
Signed-off-by: Mark Brown <broonie@kernel.org>