net: dec: tulip: de2104x: Add shutdown handler to stop NIC

net: dec: tulip: de2104x: Add shutdown handler to stop NIC

The driver does not implement a shutdown handler which leads to issues
when using kexec in certain scenarios. The NIC keeps on fetching
descriptors which gets flagged by the IOMMU with errors like this:

DMAR: DMAR:[DMA read] Request device [5e:00.0]fault addr fffff000
DMAR: DMAR:[DMA read] Request device [5e:00.0]fault addr fffff000
DMAR: DMAR:[DMA read] Request device [5e:00.0]fault addr fffff000
DMAR: DMAR:[DMA read] Request device [5e:00.0]fault addr fffff000
DMAR: DMAR:[DMA read] Request device [5e:00.0]fault addr fffff000

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20201028172125.496942-1-mdf@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>