pata_cmd64x: implement sff_irq_clear() method

Convert the driver's two bmdma_stop() methods into sff_irq_clear() methods --
they shouldn't have been bothering with clearing interrupts from the very start;
the driver will now use the standard bmdma_stop() method implementation (where
appropriate).

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>