ASoC: sgtl5000: Fix maximum value for microphone gain

sgtl5000 microphone gain only has 2 bits of resolution, so maximum value is 3.

From Eric Nelson:
"We also found that for the microphones we have here (commodity PC boom mics) a
default value of 2 for the gain gives the best results."

So change the default microphone gain as well.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>