ide: ide_timing_compute() fixup

XFER_SW_DMA_0 mode should be excluded from the extended cycle
timing computations.

[ This is just a documentation fix -- code inside the affected
  'if' block already makes sure to accept only PIO modes. ]

Noticed-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>