[libata] minor PCI IDE probe fixes and cleanups

* Replace needless 'n_ports > 2' check with a simple BUG_ON().
  No existing driver ever wants more than 2 ports.

* Delete ATA_FLAG_NO_LEGACY check.  No current driver uses
  ata_pci_init_one(), that sets this flag.

* Move PCI_CLASS_PROG register read below pci_enable_device()

* Handle ata_device_add() failure

Signed-off-by: Jeff Garzik <jeff@garzik.org>