r8169: identify different registers.

Documentation (sort of).

The location are the same, the values are the same but it is
just accidental. Note that the 810x could cope with a smaller
value as it does not support jumbo frames.

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