ASoC: sgtl5000: Fix setting mic bias resistor

ASoC: sgtl5000: Fix setting mic bias resistor

According to the datasheet:
CHIP_MIC_CTRL 0x002A

BITS[9:8] BIAS_RESISTOR
        0x0 = Powerd off
        0x1 = 2.0 kohm
        0x2 = 4.0 kohm
        0x3 = 8.0 kohm

To set mic bias resistor, we need to update bits[9:8] of
SGTL5000_CHIP_MIC_CTRL register.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>