boot: fdt: Move usable variable below updated comment

Move the variable below comment which explains what the variable means.
Update the comment. No functional change.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/boot/image-fdt.c b/boot/image-fdt.c
index c37442c..2b92bda 100644
--- a/boot/image-fdt.c
+++ b/boot/image-fdt.c
@@ -218,12 +218,12 @@
 			if (start + size < low)
 				continue;
 
-			usable = min(start + size, low + mapsize);
-
 			/*
 			 * At least part of this DRAM bank is usable, try
-			 * using it for LMB allocation.
+			 * using the DRAM bank up to 'usable' address limit
+			 * for LMB allocation.
 			 */
+			usable = min(start + size, low + mapsize);
 			addr = lmb_alloc_base(lmb, of_len, 0x1000, usable);
 			of_start = map_sysmem(addr, of_len);
 			/* Allocation succeeded, use this block. */