[PATCH] libata: create pio/atapi task queueing wrappers

Wrap pio/atapi task queueing in correspondingly named functions.  This
change doesn't change anything.  It's preparation for follow-up
pio-task/eh sync patch.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>