mfd: pm8xxx-pwm: simplify code by eliminating macros

Simplify period preprocessing to favor n=9 for big period_us.
This removes MAX_MPT here with no disadvantage. Also remove
MAX_MPT as initial min_err because it doesn't have advantage
over using maximum unsigned integer value.

Change-Id: Ie905922802ed88f322a627b1a89f4c00cc214459
Signed-off-by: Willie Ruan <wruan@codeaurora.org>