regulator: core: Properly handle the case min_uV < rdev->desc->min_uV in map_voltage_linear

Properly handle the case if the specified min_uV is less than the voltage given
by the lowest selector.

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