regulator: wm8350: Fix the logic to choose best current limit setting

Current implementation in get_isink_val actually choose the biggest current
limit setting falls within the specified range.
What we want is to choose the smallest current limit setting falls within the
specified range. Fix it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>