bnx2x: link code refactoring

Separate the interrupt setting part of each external PHY to a specific
function.
This allows calling the interrupt setting in case of link-flap avoidance,
since some link owners may not enable the interrupt on their own.

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>