iommu/mediatek: Fix handling of of_count_phandle_with_args result

The function can return negative value so it should be assigned to signed
variable. The patch changes also type of related i variable to make code
more compact and coherent.

The problem has been detected using patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>