IB/mlx4: Fix flow label returned from query QP

Correct the mask used to get the flow label, since the field is 20 bits, 
not 24 bits.

Found by Dotan Barak and Yaron Gepstein of Mellanox. 

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>