Filter out duplicate AC_PATH_REGISTERED async events.

Queued async events handling in CAM opened race, that may lead to duplicate
AC_PATH_REGISTERED events delivery during boot.  That was not happening
before r272935 because the driver was initialized later.  After that change
it started create duplicate ports in CTL.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272938 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f