ibm_newemac: Fixes entry of short packets

Short packets has to be discarded by the driver. So this patch addresses the
issue of discarding the short packets of size lesser then ethernet header
size.

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