6lowpan: ndisc: no overreact if no short address is available

6lowpan: ndisc: no overreact if no short address is available

This patch removes handling to remove short address for a neigbour entry
if RS/RA/NS/NA doesn't contain a short address. If these messages
doesn't has any short address option, the existing short address from
ndisc cache will be used. The current behaviour will set that the
neigbour doesn't has a short address anymore.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>