pata_cmd64x: use interrupt status from MRDMODE register

Start using faster version of the bmdma_stop() method for the PCI0646U and newer
chips that have the duplicate interrupt status  bits  in  the I/O mapped MRDMODE
register. Use the old, slow bmdma_stop() method on the older chips,  taking into
account that the interrupt bits are not coupled to DMA and that's enough to read
the register to clear the interrupt (on the older chips).  Determine what method
to use at the driver load time.

Fix kernel-doc of the bmdma_stop() methods, while at it.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>