bridge: multicast: add a comment to br_port_state_selection about blocking state

Add a comment to explain why we're not disabling port's multicast when it
goes in blocking state. Since there's a check in the timer's function which
bypasses the timer if the port's in blocking/disabled state, the timer will
simply expire and stop without sending more queries.

Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>