iommu/mediatek: Fix unsigned domid comparison with less than zero

Currently the check for domid < 0 is always false because domid
is unsigned. Fix this by casting domid to an int before making
the comparison.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210204150001.102672-1-colin.king@canonical.com
Addresses-Coverity: ("Unsigned comparison against 0")
Signed-off-by: Joerg Roedel <jroedel@suse.de>