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>

net/ieee802154/6lowpan/Makefile

-obj-y += ieee802154_6lowpan.o
+obj-$(CONFIG_IEEE802154_6LOWPAN) += ieee802154_6lowpan.o
 
 ieee802154_6lowpan-y := core.o rx.o reassembly.o tx.o

net/ieee802154/Makefile

 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