libata: zero xfer length on ATAPI data xfer IRQ is HSM violation

Treat zero xfer length as HSM violation.  While at it, add
unlikely()'s to ATAPI ireason and transfer length checks.

tj: Formatted patch and added unlikely()'s.

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>