r8169: fix broken register writes

This is quite similar to b39fe41f


though said registers are not even documented as 64-bit registers
- as opposed to the initial TxDescStartAddress ones - but as single
bytes which must be combined into 32 bits at the MMIO read/write
level before being merged into a 64 bit logical entity.

Credits go to Ben Hutchings <ben@decadent.org.uk> for the MAR
registers (aka "multicast is broken for ages on ARM) and to
Timo Teräs <timo.teras@iki.fi> for the MAC registers.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>