libata-scsi: be tolerant of 12-byte ATAPI commands in 16-byte CDBs

Sebastian Kemper reported that issuing CD/DVD commands under libata is
not fully compatible with ide-scsi.  In particular, the
GPCMD_SET_STREAMING was being rejected at the host level in some
instances.

The reason is that libata-scsi insists upon the cmd_len field exactly
matching the SCSI opcode being issued, whereas ide-scsi tolerates
12-byte commands contained within a 16-byte (cmd_len) CDB.

There doesn't seem to be a good reason for us to not be compatible
there, so here is a patch to fix libata-scsi to permit SCSI opcodes so
long as they fit within whatever size CDB is provided.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Tejun Heo <htejun@gmail.com>