ieee802154: 6lowpan: improve packet registration

This patch improves the packet registration handling. Instead of
registration with module init we have a open count variable and
registration the lowpan packet handler when it's needed.

The open count variable should be protected by RTNL.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>