regulator: ab8500: Fix voltage_shift setting for AB8540_LDO_DMIC

The voltage_mask is 0xc0, thus we need to set voltage_shift = 6 to get correct
selector in get_voltage_sel and set_voltage_sel.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>