mlx4: Fixing bad size of event queue buffer

We should reduce the number of reserved completion queues from the total
number of entries. Since the queue size is power of two, not reducing the
reserved entries, caused a double queue size, which may lead to allocation
failures in some cases.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>