cmd64x: interrupt status fixes (take 2)

The driver's ide_dma_test_irq() method was reading the MRDMODE register even on
PCI0643/6 where it was write-only -- fix this by always reading the "backward-
compatible" interrupt bits, renaming dma_alt_stat to irq_stat as the interrupt
status bits are not coupled to DMA.
In addition, wrong interrupt bit was tested/cleared for the primary channel --
it's bit 2 in all the chip specs and the driver used bit 1... :-/

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