bna: fix interrupt handling

This fix handles the case when IRQ handler is called (for shared IRQs)
even before the driver is ready to handle interrupts.

Signed-off-by: Debashis Dutt <ddutt@brocade.com>
Signed-off-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>