bnx2x: clamp num_queues to prevent passing a negative value

Use the clamp() macro to make the calculation of the number of queues
slightly easier to understand. It also avoids a crash when someone
accidentally passes a negative value in num_queues= module parameter.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>