r8169: failure to enable mwi should not be fatal

Few (6) network drivers enable mwi explicitly. Fewer worry about a
failure.

It is not a fix but it should avoid some annoyance like
http://bugzilla.kernel.org/show_bug.cgi?id=15454



Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Conrad Kostecki <conikost@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>