regulator: fix voltage check in regulator_is_supported_voltage()

regulator_is_supported_voltage() should return true only if the voltage
of fixed/constant regulator is between min_uV and max_uV.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org