ibm_newemac: Fixes kernel crashes when speed of cable connected changes

The descriptor pointers were not initialized to NIL values, so it was
poiniting to some random addresses which was completely invalid. This
fix takes care of initializing the descriptor to NIL values and clearing
the valid descriptors on clean ring operation.

Signed-off-by: Sathya Narayanan <sathyan@teamf1.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>