r8169: change type of argument in rtl_disable/enable_clock_request

Changing the argument type 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>