regulator: ab8500: Optimize ab8540_aux3_regulator_get_voltage_sel

We can save a register read operation in some case if read
expand_register first.
If info->expand_register.voltage_mask bit is set, no need to read
voltage_reg.

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>