qlge: Add disable/enable firmare irqs to handler.

This was accidentally omitted from one of the previous patches for firmware event
handling.  The handler needs to the enable firmware irq mask when it's done
processing or it may not get any more events interrupts.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>