bnx2: Check if_running() before touching chip registers.

Add this check to bnx2_netif_stop() and bnx2_vlan_rx_register() to
prevent bus lockups on some systems when the chip is in low power state.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>