r8169: improve rtl8169_init_ring

This function doesn't use the net_device, therefore change the
parameter to type struct rtl8169_private * to simplify the code.
In addition we don't need the calculations in the memset
statements, we can use the size of the arrays directly.

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