cmd64x: implement test_irq() method

Convert the driver's two dma_test_irq() methods into test_irq() methods. The
driver will now use the standard dma_test_irq() method implementation which
allows to remove 'cmd54x_dma_ops' and 'cmd648_dma_ops' that become identical
to 'sff_dma_ops'...

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