net: axienet: Cleanup DMA device reset and halt process

The Xilinx DMA blocks each have their own reset register, but they both
reset the entire DMA engine, so only one of them needs to be reset.

Also, when stopping the device, we need to not just command the DMA
blocks to stop, but wait for them to stop, and trigger a device reset
to ensure that they are completely stopped.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>