boot: fdt: Clean up env_get_bootm_size()

Reduce tmp variable use and remove unnecessary type cast in
env_get_bootm_size(). This aligns the env variable parsing
with env_get_bootm_low(). No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/boot/image-board.c b/boot/image-board.c
index 3263497..e3d6374 100644
--- a/boot/image-board.c
+++ b/boot/image-board.c
@@ -129,10 +129,8 @@
 	phys_addr_t start;
 	char *s = env_get("bootm_size");
 
-	if (s) {
-		tmp = (phys_size_t)simple_strtoull(s, NULL, 16);
-		return tmp;
-	}
+	if (s)
+		return simple_strtoull(s, NULL, 16);
 
 	start = gd->ram_base;
 	size = gd->ram_size;
@@ -142,7 +140,7 @@
 
 	s = env_get("bootm_low");
 	if (s)
-		tmp = (phys_size_t)simple_strtoull(s, NULL, 16);
+		tmp = simple_strtoull(s, NULL, 16);
 	else
 		tmp = start;