pwm: pca9685: Fix period change with same duty cycle

When first implementing support for changing the output frequency, an
optimization was added to continue the PWM after changing the prescaler
without having to reprogram the ON and OFF registers for the duty cycle,
in case the duty cycle stayed the same. This was flawed, because we
compared the absolute value of the duty cycle in nanoseconds instead of
the ratio to the period.

Fix the problem by removing the shortcut.

Fixes: 01ec8472 ("pwm-pca9685: Support changing the output frequency")
Cc: <stable@vger.kernel.org> # v4.3+
Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>