libata: flush is an IO command

ATA_QCFLAG_IO is used to mark commands which are used to perform
regluar IO transfers via block layer.  These commands are assumed to
be valid and taken more seriously during error handling.  Cache flush
is used by regular IO path and necessary for data integrity.  Mark it
with ATA_QCFLAG_IO.

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