[PATCH] libata: separate out ata_id_n_sectors()

Separate out n_sectors calculation into ata_id_n_sectors() from
ata_dev_identify().  This will be used by revalidation.

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