net: bridge: mst: Restrict info size queries to bridge ports

Ensure that no bridge masters are ever considered for MST info
dumping. MST states are only supported on bridge ports, not bridge
masters - which br_mst_info_size relies on.

Fixes: 122c2948

 ("net: bridge: mst: Support setting and reporting MST port states")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Link: https://lore.kernel.org/r/20220322133001.16181-1-tobias@waldekranz.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>