bna: PCI Probe Conf Lock Fix

If register_netdev() fails now, then we call mutex_unlock(&bnad->conf_mutex);
on the error path, but it's already unlocked. So we acquire the lock in error
path which will be later unlocked after the cleanup.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>