ax25: merge repeat codes in ax25_dev_device_down()

Merge repeat codes to reduce the duplication.

Signed-off-by: Lu Wei <luwei32@huawei.com>
Link: https://lore.kernel.org/r/20220516062804.254742-1-luwei32@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>