iommu: msm: zero out the bus_client in __put_bus_vote_client

The bus_client variable might become stale and re-used later. This is
causing a kernel panic when enabling bus votes on iommu v1. Fix this
by zero'ing out the variable as it's done in msm_iommu_dev-v0.c

CRs-fixed: 483497
Change-Id: Ib34a2f25d9eff0deee2e3809c6736ce2832e445f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>