soc: qcom: 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: I8f1cba6e1efa65ae41e674b14bd7598b4b521f05
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>