linuxkpi: Fix not-found case of linux_pci_find_irq_dev

Linux list_for_each_entry() does not neccessarily end with the iterator
NULL (it may be an offset from NULL if the list member is not the first
element of the member struct).

Reported by: Coverity
CID: 1366940
Reviewed by: hselasky@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8780