[forcedeth] Avoid unused-but-set variable warning in gcc 4.6

Avoid unused-but-set variable warning in gcc 4.6 which was introduced
by commit 9215b7f ("[forcedeth] Clear the MII link status register on
link status changes").

Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/forcedeth.c

 {
 	struct forcedeth_private *priv = netdev_priv ( netdev );
 	void *ioaddr = priv->mmio_addr;
-	u32 mii_status;
 
-	/* Clear the MII link change status */
-	mii_status = readl ( ioaddr + NvRegMIIStatus );
+	/* Clear the MII link change status by reading the MIIStatus register */
+	readl ( ioaddr + NvRegMIIStatus );
 	writel ( NVREG_MIISTAT_LINKCHANGE, ioaddr + NvRegMIIStatus );
 
 	if ( nv_update_linkspeed ( priv ) == 1 )