netxen: fix promisc for NX2031.

Kernel crashes, if promisc mode set without disabling rx queue.
Before changing mode in NX2031 chip, wait till rx queue drains.

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