regulator: pwm: Try to avoid voltage error in duty cycle calculation

In continuous mode of the PWM regulators, the requested voltage
PWM duty cycle is calculated in terms of 100% scale where entire
range denotes 100%. The calculation for PWM pulse ON time(duty_pulse)
is done as:

	duty_cycle = ((requested - minimum) * 100) / voltage_range.

then duty pulse is calculated as
	duty_pulse = (pwm_period/100) * duty_cycle

This leads to the calculation error if we have the requested voltage
where accurate pulse time is possible.
For example: Consider following case
	voltage range is 800000uV to 1350000uV.
	pwm-period = 1550ns (1ns time is 1mV).

	Requested 900000uV.

	duty_cycle = ((900000uV - 800000uV) * 100)/ 1550000
		   = 6.45 but we will get 6.

	duty_pulse = (1550/100) * 6 = 90 pulse time.

90 pulse time is equivalent to 90mV and this gives us pulse time equivalent
to 890000uV instead of 900000uV.

Proposing the solution in which if requested voltage makes the accurate
duty pulse then there will not be any error. On this case, if
(req_uV - min_uV) * pwm_period is perfect dividable by voltage_range
then get the duty pulse time directly.

	duty_pulse = ((900000uV - 800000uV) * 1550)/1550000)
		   = 100

and this is equivalent to 100mV and so final voltage is
(800000 + 100000) = 900000uV which is same as requested,

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