net: bridge: fix netlink export of vlan_stats_per_port option

Since the introduction of the vlan_stats_per_port option the netlink
export of it has been broken since I made a typo and used the ifla
attribute instead of the bridge option to retrieve its state.
Sysfs export is fine, only netlink export has been affected.

Fixes: 9163a0fc ("net: bridge: add support for per-port vlan stats")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>