ASoC: tas2770: Fix required DT properties in the code

ASoC: tas2770: Fix required DT properties in the code

The devicetree binding indicates that the ti,asi-format, ti,imon-slot-no
and ti,vmon-slot-no are not required but the driver requires them or it
fails to probe. Honor the binding and allow these entries to be optional
and set the corresponding values to the default values for each as defined
in the data sheet.

Fixes: 8c41a7cb58973 ("tas2770: add tas2770 smart PA kernel driver")
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200918190548.12598-4-dmurphy@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>