sfxge(4): support non-interrupting event queues creation

Poll-mode driver does not use interrupts and number of used event queues
should not be limitted by the number of interrupts allocated for the
NIC.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8967