ide: ide_id_has_flush_cache_ext() -> ata_id_flush_ext_enabled()

* Add ata_id_flush_ext_enabled() inline helper to <linux/ata.h>.

* ide_id_has_flush_cache_ext() -> ata_id_flush_ext_enabled()

  The latter one also checks if the command is marked as
  supported in word 83 and validity of words 83 & 86.

Cc: Jeff Garzik <jgarzik@pobox.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>