net/mlx4: fixing sparse warnings for not declared, functions

The SET_PORT functions are implemented in port.c, which is part
of mlx4_core, these functions are exported. The functions are in use by
the mlx4_en module (were originally part of mlx4_en).
Their declaration remained in mlx4_en module, moving the declaration to the right location.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>