I had many struggles with getting my ethernet card to work (BCM5750 14E4 ID 1677). I am attaching the KEXT that I used, you can just simply install it with KEXT HELPER INCLUDED WITH LEO4ALL. I INSTALLED AND RESTARTED. (SIDE NOTE IS I WENT INTO THE IONETWORKING KEXT and deleted BCM5751 and BCM5701 kext) before I installed kext.
Please let me know if it helps !
