ata_irq_on() isn't used outside of libata core layer.  The function is
TF/SFF interface specific but currently used by core path with some
hack too.  Move it from include/linux/libata.h to
drivers/ata/libata-sff.c.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>