net: realtek: r8169: remove redundant assignment to giga_ctrl

The variable giga_ctrl is being assigned to zero however this is
never read and hence the assignment is redundant, so remove it.
Cleans up clang warning:

drivers/net/ethernet/realtek/r8169.c:1978:3: warning: Value stored
to 'giga_ctrl' is never read
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169.c

@@ -1982,8 +1982,6 @@ static int rtl8169_set_speed_xmii(struct net_device *dev,
 		rtl_writephy(tp, MII_ADVERTISE, auto_nego);
 		rtl_writephy(tp, MII_CTRL1000, giga_ctrl);
 	} else {
-		giga_ctrl = 0;
-
 		if (speed == SPEED_10)
 			bmcr = 0;
 		else if (speed == SPEED_100)