regulator: ab8500: Use regulator_list_voltage_linear for ab8500_regulator_fixed_ops

Also removes set_voltage_time_sel callback from ab8500_regulator_fixed_ops
because the voltage won't change. ( And ab8500_regulator_fixed_ops does not
implement get_voltage_sel so set_voltage_time_sel won't be called )

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