MFC: r260058

MFC: r260058

- Remove a redundant variable in mpt_pci_attach().
- #if 0 the currently unused paired port linking and unlinking of dual
  adapters.
- Simplify MSI/MSI-X allocation and release. For a single one, we don't need
  to fiddle with the MSI/MSI-X count and pci_release_msi(9) is smart enough
  to just do nothing in case of INTx.
- Canonicalize actions taken on attach failure and detach.
- Remove the remainder of incomplete support for older FreeBSD versions.

git-svn-id: svn://svn.freebsd.org/base/stable/10@264949 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f