IXP4xx: Make mdio_bus struct static in the Ethernet driver.

Signed-off-by: NKrzysztof Hałasa <khc@pm.waw.pl>

drivers/net/arm/ixp4xx_eth.c

@@ -241,7 +241,7 @@ static inline void memcpy_swab32(u32 *dest, u32 *src, int cnt)
 
 static spinlock_t mdio_lock;
 static struct eth_regs __iomem *mdio_regs; /* mdio command and status only */
-struct mii_bus *mdio_bus;
+static struct mii_bus *mdio_bus;
 static int ports_open;
 static struct port *npe_port_tab[MAX_NPES];
 static struct dma_pool *dma_pool;