team: use netif_is_team_port()

Replace the team_port_exists() macro with its twin from netdevice.h

CC: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>