Bluetooth: 6lowpan: No need to clear memory twice

'peer_addr' is a structure embedded in 'struct lowpan_peer'. So there is no
need to explicitly call memset(0) on it. It is already zeroed by kzalloc()
when 'peer' is allocated.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>