Improve error handling around duplicate lun and port enable.

This fixes kernel panic if port enabled twice and then disabled.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274080 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Fix residual copy/paste in r274080.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274081 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f