bnx2x: Handle a rarely missed interrupt

A rare case of no link due to a missed interrupt may occur due to a
race condition between acknowledging the IGU via the BAR and restoring the NIG
interrupt mask via the GRC.
To solve it, we wait for the IGU ack command to finish prior to restoring the
NIG interrupt mask.

Signed-off-by: Yaniv Rosner <yaniv.rosner@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>