bnx2x: Refactor MAC address setup code.

For iSCSI MAC address setup in later patches.

Signed-off-by: Shmulik Ravid - Rabinovitz <shmulikr@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>