[PATCH] libata: separate out ata_id_major_version()

Separate out ATA major version calculation from ata_dev_identify()
into ata_id_major_version().  It's preparation for splitting
ata_dev_identify().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>