fakelb: don't deliver when one phy

A real phy don't transmit the transmitted frame back to the phy. This
behaviour will confuse the 6LoWPAN and the upper IPv6 neighbour
discovery cache. We need now at least two virtual phy's to creating a
virtual wpan network.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>