regulator: 88pm800: fix LDO vsel_mask value

As per datasheet,
Except LDO2, all other LDO's use bit [3:0] for VOUT select.

Current code uses wrong mask value of 0x1f, So this patch
fixes it to use 0xf.

Signed-off-by: Yi Zhang <yizhang@marvell.com>
[vaibhav.hiremath@linaro.org: Updated changelog with more detailed description]
Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>