ieee802154: 6lowpan: fix Makefile entry

Since commit ea81ac2e7050798109356150ea16e71622a5c329 ("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>
diff --git a/net/ieee802154/Makefile b/net/ieee802154/Makefile
index 3653397..05dab29 100644
--- a/net/ieee802154/Makefile
+++ b/net/ieee802154/Makefile
@@ -1,6 +1,6 @@
 obj-$(CONFIG_IEEE802154) += ieee802154.o
 obj-$(CONFIG_IEEE802154_SOCKET) += ieee802154_socket.o
-obj-$(CONFIG_IEEE802154_6LOWPAN) += 6lowpan/
+obj-y += 6lowpan/
 
 ieee802154-y := netlink.o nl-mac.o nl-phy.o nl_policy.o core.o \
                 header_ops.o sysfs.o nl802154.o