ASoC: TWL4030: Fix for digital loopback gain range

When the gain is configured using dB value it was
not possible to use -24dB since the loopback got
muted instead of -24dB.

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