r8169: fix Config2 MSIEnable bit setting.

The MSIEnable bit is only available for the 8169.

Avoid Config2 writes for the post-8169 8168 and 810x.

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