bridge: multicast: start querier timer when running user-space stp

When STP is running in user-space and querier is configured, the
querier timer is not started when a port goes to a non-blocking state.
This patch unifies the user- and kernel-space stp multicast port enable
path and enables it in all states different from blocking. Note that when a
port goes in BR_STATE_DISABLED it's not enabled because that is handled
in the beginning of the port list loop.

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>