r8169: corrupted IP fragments fix for large mtu.

Noticed with the 8168d (-vb-gr, aka RTL_GIGA_MAC_VER_26).

ConfigX registers should only be written while the Config9346 lock
is held.

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