regulator: wm831x-dcdc: Fix the logic to choose best current limit setting

Current code in wm831x_buckv_set_current_limit actually set the current limit
setting greater than specified range.

Fix the logic in wm831x_buckv_set_current_limit to choose the
smallest current limit setting falls within the specified range.

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