6lowpan: fix hop limit compression

Add missing pointer shift for the 'default' case.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Cc: Tony Cheneau <tony.cheneau+zigbeedev@amnesiak.org>
Signed-off-by: David S. Miller <davem@davemloft.net>