msm: msm_bus: Set current index to default value

When a client unregisters with bus driver after registering with it,
without a bandwidth update request in-between, the current index is
not having valid value, so update it to default value of 0.

Change-Id: I03d691fd4bbfc557e580233cc9075c027a8ef177
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>