datapath: Fix parsing invalid LLC/SNAP ethertypes

Before this patch, if an LLC/SNAP packet with OUI 00:00:00 had an ethertype
less than 1536 the flow key given to userspace in the upcall would contain the
invalid ethertype (for example, 3). If userspace attempted to insert a kernel
flow for this key it would be rejected by ovs_flow_from_nlattrs.

This patch allows OVS to pass the OFTest pktact.DirectBadLlcPackets.

Signed-off-by: Rich Lane <rlane@bigswitch.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>