net/mlx5e: Make function mlx5e_change_rep_mtu() static

Fixes the following sparse warning:

drivers/net/ethernet/mellanox/mlx5/core/en_rep.c:903:5: warning:
 symbol 'mlx5e_change_rep_mtu' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>