libata: add ATAPI_* cmd types and implement atapi_cmd_type()

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