net: axienet: Clean up DMA start/stop and error handling

[ Upstream commit 84b9ccc0 ]

Simplify the DMA error handling process, and remove some duplicated code
between the DMA error handling and the stop function.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 5a6caa2c ("net: xilinx: axienet: Fix packet counting")
Signed-off-by: Sasha Levin <sashal@kernel.org>