iommu/arm-smmu: fix a signedness bug

Unsigned char is never equal to -1.

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>