ARM: tegra: dvfs: Fix error checking of voltage

Fix error checking of predicted voltage. Also get rid of
maybe-uninitialized warning when using 4.6 toolchain.

Bug 949219
Bug 999222

Change-Id: I47553aba5a93c91bdd93cbf75081d69f92aec4dd
Signed-off-by: Juha Tukkinen <jtukkinen@nvidia.com>
Reviewed-on: http://git-master/r/108899
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>