regulator: pwm: Fix calculation of voltage-to-duty cycle

With following equation for calculating
voltage_to_duty_cycle_percentage
	100 - (((req_uV * 100) - (min_uV * 100)) / diff);

we get 0% for max_uV and 100% for min_uV.

Correcting this to
	((req_uV * 100) - (min_uV * 100)) / diff;
 to get proper duty cycle.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>