cmd64x: set IDE_HFLAG_SERIALIZE explictly for CMD646

* Set IDE_HFLAG_SERIALIZE explictly for CMD646.

* Remove no longer needed ide_cmd646 chipset type (which has
  a nice side-effect of fixing handling of unexpected IRQs).

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