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

Current code in wm831x_isink_set_current actually set the current limit setting
smaller than specified range.

Fix the logic in wm831x_isink_set_current 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>