cmd_usage: constify

The usage helper doesn't modify the command, so constify its input arg.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/common/command.c b/common/command.c
index b3ec510..ddaed68 100644
--- a/common/command.c
+++ b/common/command.c
@@ -140,7 +140,7 @@
 	return find_cmd_tbl(cmd, &__u_boot_cmd_start, len);
 }
 
-int cmd_usage(cmd_tbl_t *cmdtp)
+int cmd_usage(const cmd_tbl_t *cmdtp)
 {
 	printf("%s - %s\n\n", cmdtp->name, cmdtp->usage);