netxen: protect resource cleanup by rtnl lock

o context resources can be in used, while resource cleanup is in progress,
  during fw recover.
o Null pointer execption can occur in send_cmd_desc, if fw recovery
  module frees tx ring without rtnl lock.
o Same applies to ethtool register dump.

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