regulator: spm-regulator: Fix the usage of the 'uV' variable

The commit 639277b6c73c ("regulator: spm-regulator: Add additional
settling delay for  FTS2.5 SMPS") incorrectly changed the usage
of 'uV' variable. The local 'uV' variable holds the correct voltage
level to be scaled up/down. Use it instead of vreg->uV.

CRs-Fixed: 1036738
Change-Id: I52540237a4db79c149409c6017ffc750b5abddd2
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>