ieee802154: 6lowpan: fix Makefile entry

Since commit ea81ac2e ("ieee802154:
create 6lowpan sub-directory") we have a subdirectory for the ieee802154
6lowpan implementation. This commit also moves the Kconfig entry inside
of net/ieee802154/6lowpan/ and forgot to rename the Makefile entry from
obj-$(CONFIG_IEEE802154_6LOWPAN) to obj-y and handle the
obj-$(CONFIG_IEEE802154_6LOWPAN) inside the created 6lowpan directory.
This will occur that the ieee802154_6lowpan can't be build.

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