bridge: ignore bogus STP config packets

If the message_age is already greater than the max_age, then the
BPDU is bogus. Linux won't generate BPDU, but conformance tester
or buggy implementation might.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>