power: vm-bms: Use the correct variable to compare shutdown_soc

Shutdown SOC is stored in the driver structure variable. Use
the same for shutdown-soc-valid comparison.

CRs-Fixed: 717658
Change-Id: I73cb2e19973e6326ccf3d9935a52905b21013678
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>