net/mlx4: Add a check if there are too many reserved QPs

The number of reserved QPs is affected both from the firmware and
from the driver's requirements. This patch adds a check that
validates that this number is indeed feasable.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>