iommu/mediatek: simplify dt parsing error handling

As previously documented by commit 26593928564c ("iommu/mediatek: Add
error path for loop of mm_dts_parse"), the id mapping may not be linear
so the whole larb array needs to be iterated on devicetree parsing
errors.

Simplify the loop by iterating from index zero while dropping the
redundant NULL check for consistency with later cleanups.

Also add back the comment which was removed by commit 462e768b55a2
("iommu/mediatek: Fix forever loop in error handling") to prevent anyone
from trying to optimise the loop by iterating backwards from 'i'.

Cc: Yong Wu <[e-mail]>
Acked-by: Robin Murphy <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Reviewed-by: Yong Wu <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Signed-off-by: Joerg Roedel <[e-mail]>

drivers/iommu/mtk_iommu.c

···
 	return 0;
 
 err_larbdev_put:
-	for (i = MTK_LARB_NR_MAX - 1; i >= 0; i--) {
-		if (!data->larb_imu[i].dev)
-			continue;
+	/* id mapping may not be linear, loop the whole array */
+	for (i = 0; i < MTK_LARB_NR_MAX; i++)
 		put_device(data->larb_imu[i].dev);
-	}
+
 	return ret;
 }