iommu: dma-mapping-fast: fix iova alloc wrap

In case of iova_alloc bitmap search wrap, we erroneously update the
previous_next_start as 0, this causes stale tlb logic not detect wrap and
stale TLB entry issues. Fix the same.

Change-Id: Ic8c96e98abc91520f222a7dbe5024814dabd5042
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>