ASoC: tlv320dac33: Change magic numbers used in Mode7

Upper and Lower threshold values are used as magic
numbers. Replace them with defines for later use.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>