ide: idedisk_supports_lba48() -> ata_id_lba48_enabled()

* Add ata_id_lba48_enabled() inline helper to <linux/ata.h>.

* idedisk_supports_lba48() -> ata_id_lba48_enabled()

  The latter one also checks validity of words 83 & 86.

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