bridge: Fix timer typo that may render snooping less effective

In a couple of spots where we are supposed to modify the port
group timer (p->timer) we instead modify the bridge interface
group timer (mp->timer).

The effect of this is mostly harmless.  However, it can cause
port subscriptions to be longer than they should be, thus making
snooping less effective.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>