ENGR00242214 IMX PWM:Correct duty cycle calculation

ENGR00242214 IMX PWM:Correct duty cycle calculation

Since we've already reduced 2 cycles before writing PWMPR
register, the real period cycle on PWMO is the value of
period_cycles (before reducing 2). So, the following commit
message of ENGR00170342, which changes the duty cycle
calculation wrongly, is not reasonable:
===================================================
The chip document says the counter counts up to period_cycles + 1
and then is reset to 0, so the actual period of the PWM wave is
period_cycles + 2
===================================================

Revert "ENGR00170342 PWM: fix pwm output can't be set to 100% full duty"

This reverts commit ac3711f7f24b94db9f78fd7e9bf134c2ecd025ab.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
(cherry picked from commit b42be77aa7842834f0fb50924546701b668d7ab9)