msm: avs: Restore core voltage when disabling AVS

AVS changes the core voltage to match the current operating conditions
on the part. Not all frequencies supported by the target have characterized
AVS delay synthesizer (AVSDSCR) values. For frequencies that do not have
AVSDSCR set, AVS is not enabled and so need to operate at the
characterized voltage for that frequency. Disabling AVS does not
however, restore the operating voltage to the value the rail was
configured to. This may cause frequencies that do not have AVSDSCR to
run at lower voltage than prescibed.
Set up the core voltage back to the last known good value when disabling
AVS.

Change-Id: I5389996288fa49e45c469d2a8917e00900261de7
CRs-fixed: 457772
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>