net/mlx4: fix sparse warnings on TX control flags, endianess

Fix sparse warnings on incompatibility between the endianess of the ctrl_flags
field of struct mlx4_en_priv to the srcrb_flags field of struct
mlx4_wqe_ctrl_seg by changing the former to be __be32 instead of u32.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>