netxen: Fix interrupt handling for multiport adapters

This patch fixes masking of interrupts on multiport adapters. Also disables
interrupts upon ifdown interface. The wrong mask could result in interrupt
flood after interface is down.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>