libata-acpi: improve ACPI disabling

* If _GTF evalution fails, it's pointless to retry.  If nothing else
  is wrong, just ignore the error.

* After disabling ACPI, return success iff the number of executed _GTF
  command equals zero.  Otherwise, tell EH to retry.  This change
  fixes bogus 1 return bug where ata_acpi_on_devcfg() expects the
  caller to reload IDENTIFY data and continue but the caller
  interprets it as an error.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>