6lowpan: set and use mac_len for mac header length

Set the mac header length while creating the 802.15.4 mac header.

Drop the function for recalculate mac header length in upper layers
which was static and works for intra pan communication only.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>