net: ravb: staticize ravb_start

Cosmetic fix, make ravb_start() static.
Signed-off-by: NMarek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>

drivers/net/ravb.c

@@ -438,7 +438,7 @@ static int ravb_config(struct udevice *dev)
 	return 0;
 }
 
-int ravb_start(struct udevice *dev)
+static int ravb_start(struct udevice *dev)
 {
 	struct ravb_priv *eth = dev_get_priv(dev);
 	int ret;