iommu: arm-smmu: Avoid register writes on multiple detach calls

Two or more racing detach calls are possible, where the attach_count
gets decremented multiple times and becomes negative, due to which
the necessary check would fail and register writes would occur followed
by TLB flush all. Check if attach_count was zero, and return if so.

Change-Id: I0f33585c57f4d96e475c672604bb9163edc332fb
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>