regulator: anatop: Fix wrong mask used in anatop_get_voltage_sel

The mask used in anatop_get_voltage_sel does not match the mask used in
anatop_set_voltage_sel.
We need to do left shift anatop_reg->vol_bit_shift bits for the correct mask.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>