ixl: Fix panic caused by bug exposed by r344062

Don't use a struct if_irq for IFLIB_INTR_IOV type interrupts since that results
in get_core_offset() being called on them, and get_core_offset() doesn't
handle IFLIB_INTR_IOV type interrupts, which results in an assert() being triggered
in iflib_irq_set_affinity().

PR: 235730
Reported by: Jeffrey Pieper <jeffrey.e.pieper@intel.com>
MFC after: 1 day
Sponsored by: Intel Corporation