pata_platform: don't use generic ata_port_start

I have a system where I have a simple IDE controller that sits on a
local bus without bus master dma capability, and thus no dma_mapping
ops defined for the device/bus.

pata_platform works great for me, with the exception of using the generic
ata_port_start which tries to do a dmam_alloc_coherent.

Looks like it doesn't need to allocate a prd table at all, so replace it
with a dummy function instead.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>