regulator: pwm-regulator: Small clean-ups

Remove over-bracketing, use framework API to fetch PWM period and
be more forthcoming that pwm_voltage_to_duty_cycle() actually returns
duty cycle as a percentage, rather than a register value.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>