AHCI - properly initialize ccb_xa.flags in softreset, make poll code robust

AHCI - properly initialize ccb_xa.flags in softreset, make poll code robust

* The softreset code did not properly initialize ccb_xa.flags, causing
  the softreset FIS's to sometimes get queued as an NCQ command instead of
  as a non-NCQ command.

* Make ahci_poll() a bit more robust.  Properly set ccb_xa.state on
  timeout, check for unexpected completions, and check to see if the
  ccb was put on a queue (though the latter should never happen since
  active/sactive is cleared by ahci_get_err_ccb()).