cmd64x: implement clear_irq() method (take 2)

Convert the driver's two dma_end() methods into clear_irq() methods -- the
driver will now use the standard dma_end() method implementation, ide_dma_end().

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>