r8169: change type of first argument in rtl_tx_performance_tweak

Changing the type of the first argument to struct rtl8169_private * is more
in line with the other functions in the driver and it allows to reduce the
code size.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>