gianfar: pass the proper dev to DMA ops

We need to be passing the of_platform device struct into the DMA ops as
its the one that has the archdata setup to know which low-level DMA ops we
should be using (not the net_device one).  This isn't an issue until we
expect the archdata to be setup correctly.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>