r8169: Add software counter for multicast packages

The multicast hardware counter on 8168/8111 chips is only 32 bit while the
statistics in struct rtnl_link_stats64 are 64 bit.  Given that statistics
are requested on an irregular basis, an overflow of the hardware counter
can go unnoticed.  To count even very large numbers of multicast packets
reliably, add a software counter and remove previously applied code to
fill the multicast field requested by @rtl8169_get_stats64 with the values
read from the rx_multicast hardware counter.

Signed-off-by: Corinna Vinschen <vinschen@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>