mlx4_core: Fix CQ context layout

The reserved6 field should be 64 bits, not just 16 bits.  Without
this, the structure does not match the hardware layout on 32-bit
architectures: the db_rec_addr field ends up at offset 52 instead of
offset 56.  The bug slipped by because the alignment of __be64 members
ends up putting it in the right place on x86-64.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>