r8169: multicast register update

The layout of the 8101 series is identical to that of the 8168 one,
thus allowing to pack everything not 8169 related above MAC_VER_06.
New 810x and 8168 chipsets should automagically behave correctly.

It matches code in Realtek's 1.008.00 8101 and 8.007.00 8168 drivers.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>