bna: Fix warning false positive.

GCC can't see that in all non-error-return paths we do in fact
set *using_dac to something.

Add an explicit initialization to remove this warning:

drivers/net/ethernet/brocade/bna/bnad.c: In function ‘bnad_pci_probe’:
drivers/net/ethernet/brocade/bna/bnad.c:3079:5: warning: ‘using_dac’ may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/net/ethernet/brocade/bna/bnad.c:3233:7: note: ‘using_dac’ was declared here

Signed-off-by: David S. Miller <davem@davemloft.net>