The SCSI T10 committee is working on a document describing a standard
method for translating ATA<->SCSI, since it being done quite often
these days.  Some of the recommendations are reasonable, and we implement
two here:

* Mirror that ATA 'removeable media' bit into INQUIRY output.
* Change behavior of INQUIRY output field 'product revision' from
  the libata software version number to the first 4 bytes of the
  ATA device's firmware revision number.

Rather than cache the firmware revision in struct ata_device, as was/is
done with two other strings, I took the opportunity to eliminate the
caching of the two other strings, 'vendor' and 'product'.  These strings
are now retrieved as needed from the IDENTIFY [PACKET] DEVICE info page,
since we cache its entire contents.

Retrieving a string from the identify-device page is done via the
helper function ata_dev_id_string(), which is now exported.

This patch winds up making struct ata_device 40 bytes smaller, and
the libata core gets a bit smaller as well.