libata: implement HDIO_GET_IDENTITY

'hdparm -I' doesn't work with ATAPI devices and sg_sat is not widely
spread yet leaving no easy way to access ATAPI IDENTIFY data.
Implement HDIO_GET_IDENTITY such that at least 'hdparm -i' works.

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