ixl: silence runtime warning when PCI_IOV is not enabled

When PCI_IOV is not enabled, do not attempt to call
iflib_softirq_alloc_generic(...IFLIB_INTR_IOV), as it results
in boot-time warnings similar to:
taskqgroup_attach_cpu: qid not found for iov cpu=2
ixl2: taskqgroup_attach_cpu failed 22
Instead, make it conditional on PCI_IOV like the other
SR-IOV related code.

Reviewed by: erj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37609