cmd64x: use interrupt status from MRDMODE register (take 2)

Fold the parts of the ide_dma_end() methods identical to __ide_dma_end() into a
mere call to it.
Start using faster versions of the ide_dma_end() and ide_dma_test_irq() methods
for the PCI0646U and newer chips that have the duplicate interrupt status bits
in the I/O mapped MRDMODE register, determing what methods to use at the driver
load time. Do some cleanup/renaming in the "old" ide_dma_test_irq() method too.

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