msm: vp: Let get_voltage retrieve voltage at PMIC

Earlier get_voltage implementation was returning the cached
value of last set_voltage call. This will become problematic
since current DCVS driver acpuclock is not using this
regulator interface and do set the GO bit for the setting
the new voltage from the current PLEVEL0..7 registers, so the
cached value returned by get_voltage may not be the current
voltage at the PMIC.

Change-Id: Ib10bde6264dfaaf1f84c6ca84bcbcffb1c9f37d7
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>