[libata] minor fixes, new helpers

- in ata_dev_identify(), don't assume that all devices are either
  ATA or ATAPI.  In the future, this code will see port multipliers
  and other devices.
- make a debugging printk less verbose
- add new helper ata_qc_reinit()
- add new helper BPRINTK() and port flag ATA_FLAG_DEBUGMSG, for
  fine-grained debugging use.