fec_mxc.c: Fix MX27 FEC MAC validity check

Fix MX27 FEC logic to check validity of the MAC address in fuse.
Only null (empty fuse) or invalid MAC address was retrieved from mx27 fuses before this change.

Signed-off-by: Eric Jarrige <jorasse@armadeus.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>