ide: move IRQ clearing from ack_intr() method to clear_irq() method (take 2)

There are now two methods that clear the port interrupt: ack_intr() method,
implemented only on M680x0 machines, that is called at the start of ide_intr(),
and clear_irq() method, that is called somewhat later in this function.  In
order to stop this duplication, delegate the task of clearing the interrupt
to clear_irq() method, only leaving to ack_intr() the task of testing for the
port interrupt.

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