regulator: core: Handle fixed voltage in map_voltage_linear

Fixed voltage is a kind of linear mapping where n_voltages is 1.
This change allows [list|map]_voltage_linear to be used for fixed
voltage.

For fixed voltage, n_voltages is 1 and the only valid selector is 0.
Thus we actually don't care the uV_step setting.

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