net/mlx4_en: Fix setting initial MAC address

Commit 6bbb6d99

 "net/mlx4_en: Optimize Rx fast path filter checks" introduced a regression
under which the MAC address read from the card was not converted correctly
(the most significant byte was not handled), fix that.

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