regulator: pwm-regulator: Calculate the output voltage for disabled PWMs

If a PWM output is disabled then it's voltage has to be calculated
based on a zero duty cycle (for normal polarity) or duty cycle being
equal to the PWM period (for inverted polarity). Add support for this
to pwm_regulator_get_voltage().

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://msgid.link/r/20240113224628.377993-3-martin.blumenstingl@googlemail.com
Signed-off-by: Mark Brown <broonie@kernel.org>