power: vm-bms: Fixes for the BMS control path

1. Reject fifo_data if fifo_count is > 8.
2. Increase the fsm state switch delay to 500us.
3. Hold a mutex while updating fifo_length.
4. Apply master-control while updating fifo_length.

CRs-Fixed: 661267
Change-Id: I250fcb0c0aeefc2765593c5f1641281e9566c7bc
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>