ata_qc_issue() BUG_ON()s on data commands w/o data, which may be
submitted via SG_IO.  Be less of a drama queen and just trigger
WARN_ON_ONCE() and fail the command with AC_ERR_SYSTEM.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NStefan Hübner <stefan.huebner@stud.tu-ilmenau.de>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>