team: Publish team_port_get_rcu()

A follow-up patch adds a new entry point, team_port_dev_txable(). Making
it an ordinary exported function would mean that any module that may
need the service in one of the supported configurations also
unconditionally needs to pull in the team module, whether or not the
user actually intends to create team interfaces.

To prevent that, team_port_dev_txable() is defined in if_team.h, and
therefore all dependencies of that function also need to be
publicly-visible.

Therefore move team_port_get_rcu() from team.c to if_team.h.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>