Fix missing return in do_mem_loop()

For some reason this does not normally cause a compiler warning, but the code
seems to be incorrect. Add the missing return.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_mem.c b/common/cmd_mem.c
index 6df00b1..77eafa0 100644
--- a/common/cmd_mem.c
+++ b/common/cmd_mem.c
@@ -551,6 +551,8 @@
 			*cp++;
 	}
 	unmap_sysmem(buf);
+
+	return 0;
 }
 
 #ifdef CONFIG_LOOPW