ieee802154: 6lowpan: remove EXPORT_SYMBOL

This function is used internally inside of ieee802154 6lowpan module
only and not outside of any other module. We don't need to export this
function then.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>