lan78xx: Simple patch to prevent some crashes

Alexander Graf submitted a patch to solve a problem when connecting to
the device before the interface is brought up, but with that patch
applied the interface fails to work (for me and others). This simpler
patch should be considered a stepping stone that may prevent some of the
crashes seen when the interface is reset but without solving the early
access problem.

See:  https://github.com/raspberrypi/linux/issues/2449



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