regulator: pwm-regulator: Simplify voltage to duty-cycle call

If we reverse some of the logic and change the formula used,
we can simplify the function greatly.

It is intentional that this function is supplied and then re-worked
within the same patch-set.  The submission in the previous patch is
the tried and tested (i.e. in real releases) method written by ST.
This patch contains a simplification provided later.  It looks and
performs better, but doesn't have the same time-under-test that the
original method does.  The idea is that we keep some history in
order to provide an easy way back i.e. revert.

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