bna: use container_of to resolve bufdesc_ex from bufdesc

Use container_of instead of casting first structure member.

Compiled but untested.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/ethernet/brocade/bna/bna_enet.c
Simple merge

diff --cc drivers/net/ethernet/brocade/bna/bna_tx_rx.c
Simple merge