r8169: Enable RX_MULTI_EN for RTL_GIGA_MAC_VER_40

The ethernet port on my ASUS A88X Pro mainboard stopped working
several times a day, with messages like these in dmesg:

AMD-Vi: Event logged [IO_PAGE_FAULT device=05:00.0 domain=0x001e address=0x0000000000003000 flags=0x0050]

Searching the web for these messages led me to similar reports about
different hardware supported by r8169, and eventually to commits
3ced8c95 ('r8169: enforce RX_MULTI_EN
for the 8168f.') and eb2dc35d ('r8169:
RxConfig hack for the 8168evl'). So I tried this change, and it fixes
the problem for me.

Signed-off-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: David S. Miller <davem@davemloft.net>