netfilter: bridge: clarify bridge/netfilter message

When using bridge without bridge netfilter enabled the message
displayed is rather confusing and leads to belive that a deprecated
feature is in use. Use IS_MODULE to be explicit that the message only
affects users which use bridge netfilter as module and reword the
message.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>