mlx4_en: remove sparse errors

Fix new sparse errors introduced in commit 62212171 (mlx4_en: dont
change mac_header on xmit)

Reported-by: Or Gerlitz <or.gerlitz@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>