power: vm-bms: Fix a bug while re-enabling BMS

Set the correct bit(7) to enable BMS.

While at it, reject the OCV if it is < 2V. This is to handle the
runtime battery-swap usecase where the BMS is disabled and re-enabled
to clear old data. In such a case OCV_DATA is reset to 0, which messes
the SOC calculation.

Change-Id: I83c21d0aca45eeaa3fd4587174bb597883fd445a
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>