lp3971: Fix BUCK_VOL_CHANGE_SHIFT logic

Given x=0,1,2, current implementation of BUCK_VOL_CHANGE_SHIFT(x) returns 0,4,8.
The correct return value should be 0,4,6.
This patch fix the logic.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>