Fix missing path conversion from the previous commit to shuffle mdio around.

It turns out the recent work to cut down the number of atheros kernels built
didnt include one with ARGE_MDIO defined..

sys/mips/atheros/if_arge.c

@@ -78,7 +78,7 @@ __FBSDID("$FreeBSD$");
 #include "opt_arge.h"
 
 #if defined(ARGE_MDIO)
-#include <dev/etherswitch/mdio.h>
+#include <dev/mdio/mdio.h>
 #include <dev/etherswitch/miiproxy.h>
 #include "mdio_if.h"
 #endif