msm: msm_bus: Check for invalid current voting index

Check for negative values in current voting index when attempting
to unregister a device and default to 0. This can happen when clients
have yet to make a vote after registration and attempt to unregister
right away.

Change-Id: I8a384a98984964756c569d09be4bb3a0614b475c
Signed-off-by: David Dai <daidavid1@codeaurora.org>