bna: change type of bna_id to atomic_t

Change type of bna_id to atomic_t. The bnad_list_mutex is used to prevent
a race when bna_id is incremented. After the change the mutex can be
removed in the next step.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>