bnx2x: Clean previous IGU status before ack

When enabling interrupts, acknowledge the interrupt only
after configuring the IGU to the correct interrupt mode
(otherwise it would dirty selftests)

Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>