6lowpan: Set MAC address length according to LOWPAN_LLTYPE

Set MAC address length according to the 6LoWPAN link layer in use.
Bluetooth Low Energy uses 48 bit addressing while IEEE802.15.4 uses
64 bits.

Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>