Cope with the fact that a failure in iflib_device_register() can result
in em_free_pci_resources() being called after receive queues have
already been freed. In particular, a failure to allocate MSI-X IRQ
resources will goto fail_queues, where IFDI_QUEUES_FREE() will be called
via iflib_tx_structures_free(), preceding the call to IFDI_DETACH().
A similar check is present in ixgbe(4) and ixl(4).