iommu/arm-smmu: fix a signedness bug

Unsigned char is never equal to -1.

Change-Id: I9c55130c5dc76cacea2bf79105fd884043e2e6eb
Cc: <stable@vger.kernel.org>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: faea13b72dbdb77e4d6ad83344596486611708b0
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>