MFC r305869:
mlx5en: Minor completion queue control path code refactor.

Move setting of CQ moderation mode together with the other
CQ moderation parameters. Pass completion event vector as
a separate argument to mlx5e_open_cq(), because its value is
different for each call. Pass mlx5e_priv pointer instead of
mlx5e_channel pointer so that code can be used by rate
limiting sendqueues.

Sponsored by: Mellanox Technologies