6lowpan: obtain IEEE802.15.4 sequence number from the MAC layer

Sets the sequence number in the frame format. Without this fix, the sequence
number is always set to 0. This makes trafic analysis very hard.

Signed-off-by: Tony Cheneau <tony.cheneau@amnesiak.org>
Signed-off-by: David S. Miller <davem@davemloft.net>