cxgbe(4): Do not forward interrupts to queues with freelists. This
leaves the firmware event queue (fwq) as the only queue that can take
interrupts for others.

This simplifies cfg_itype_and_nqueues and queue allocation in the driver
at the cost of a little (never?) used configuration. It also allows
service_iq to be split into two specialized variants in the future.

MFC after: 2 months
Sponsored by: Chelsio Communications