lan78xx: Ignore DT MAC address if already valid

The patch to set the lan78xx MAC address from DT does so regardless of
whether or not the interface already has a valid address. As the
initialisation function is called from the reset handler when the
interface is brought up, it is impossible to change the MAC address
in a way that persists across the interface being brought up.

Fix the problem by moving the DT reading code after the check for a
valid address.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=209309



Signed-off-by: Phil Elwell <phil@raspberrypi.org>