lp3971: Fix setting val for LDO2 and LDO4

In lp3971_ldo_set_voltage function, it requires val to left shift 4
bits for LDO2 and LDO4.
This patch fix this issue.

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>