mlx4_core: fix wrong comment about the reason of subtract one from the max_cqes

The reason for the pre-allocation of one CQE is to enable resizing of
the CQ.
Fix comment accordingly.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.com>
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>