iommu: msm: refine the check for double attach cases

MSM IOMMU driver wants to check if any dynamic domain is
getting double attach or not. For the same, present check
is wrong and gives -EBUSY for the very first dynamic_attach
as by default asid field can be 0. Fix this bug by re-writing
the condition properly.

Change-Id: I658de5630ac61d3e0a1c0c9a14854b47b1f6adee
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>