regulator: core: Make set_voltage_tol() try for mid-range first

The expected semantic for something expressed as a tolerance is that it
should deliver the specified value with some deviation allowed but this
is not what set_voltage_tol() currently does. Instead it just passes
the maximum possible range to set_voltage() which will typically result
in a voltage aimed at lower than the target voltage.

Instead first try to set a voltage between the target voltage and the
upper limit, then fall back on the full range. This will be much more
robust against physical variation in systems and makes the API behave
more like users would expect.

Signed-off-by: Mark Brown <broonie@linaro.org>