net: bridge: fix uninitialized variables when BRIDGE_CFM is disabled

Function br_get_link_af_size_filtered() calls br_cfm_{,peer}_mep_count()
that return a count. When BRIDGE_CFM is not enabled these functions
simply return -EOPNOTSUPP but do not modify count parameter and
calling function then works with uninitialized variables.
Modify these inline functions to return zero in count parameter.

Fixes: b6d0425b ("bridge: cfm: Netlink Notifications.")
Cc: Henrik Bjoernlund <henrik.bjoernlund@microchip.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>