libata: Add special ata_pio_need_iordy() handling for Compact Flash.

According to the Compact Flash specification r4.1, PIO modes 5 and 6
do not use iordy.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>