regulator: Fix set and get current limit for wm831x_buckv

WM831X_DC1_HC_THR_MASK is 0x0070.
We need to do proper shift for setting and getting current limit.

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