Add ATAPI command types - ATAPI_READ, WRITE, RW_BUF, READ_CD and MISC,
and implement atapi_cmd_type() which takes SCSI opcode and returns to
which class the opcode belongs.  This will be used later to improve
ATAPI handling.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>