miiphy: constify device name

The driver name does not need to be writable, so constify it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
diff --git a/common/cmd_mii.c b/common/cmd_mii.c
index bb94186..3ea493b 100644
--- a/common/cmd_mii.c
+++ b/common/cmd_mii.c
@@ -299,7 +299,7 @@
 	unsigned char	addr, reg;
 	unsigned short	data;
 	int		rcode = 0;
-	char		*devname;
+	const char	*devname;
 
 	if (argc < 2)
 		return cmd_usage(cmdtp);