netxen: fix inconsistent lock state

Spin lock rds_ring->lock is used in poll routine, so other users should
use spin_lock_bh(). While posting rx buffers from netxen_nic_attach,
rds_ring->lock is not required, so cleaning it instead of fixing it by
spin_lock_bh().

Signed-off-by: Amit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>