regulator: qcom_pm8008: Fix regulator_get_voltage API

Currently the LDO voltage is read from "VSET_VALID_LB" and
"VSET_VALID_UB" registers which provides the actual voltage
when the LDO is enabled. However, these registers return a
value "0" when the LDO is disabled. Due to this the incorrect
voltage is set on the parent buck which leads to voltage
ripples. Fix it by reading VSET_LB and VSET_UB registers
which return a valid voltage even though the LDO is disabled.

Change-Id: Id47882802a1d39f7843a40853edfc4f6d17376f5
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>