Make isp_find_pdb_by_*() search for targets in portdb in reverse order.

Records with target_mode == 1 are allocated from the end of portdb, so it
seems logical to start search from the end not traverse whole array.

MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275112 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f