ASoC: tas6424: Allow disabling auto diagnostics for faster power-on

The TAS6424 incorporates both DC-load and AC-load diagnostics which are
used to determine the status of the load. The DC diagnostics runs when any
channel is directed to leave the Hi-Z state and enter the MUTE or PLAY
state.
The DC diagnostics are turned on by default but, if a fast startup without
diagnostics is required, the diagnostics can be disabled using a dedicated
ALSA control.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>