net: xilinx: axienet: Fix race in axienet_stop

axienet_dma_err_handler can race with axienet_stop in the following
manner:

CPU 1                       CPU 2
======================      ==================
axienet_stop()
    napi_disable()
    axienet_dma_stop()
                            axienet_dma_err_handler()
                                napi_disable()
                                axienet_dma_stop()
                                axienet_dma_start()
                                napi_enable()
    cancel_work_sync()
    free_irq()

Fix this by setting a flag in axienet_stop telling
axienet_dma_err_handler not to bother doing anything. I chose not to use
disable_work_sync to allow for easier backporting.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Fixes: 8a3b7a25 ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
Link: https://patch.msgid.link/20240903175141.4132898-1-sean.anderson@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>