ieee802154: 6lowpan: fix drop return value

This patch changes the return value of lowpan packet receive handler to
the correct NET_RX_DROP instead RX_DROP.

This issue was detected by sparse and reported from Marcel:
net/ieee802154/6lowpan/rx.c:329:32:    expected int
net/ieee802154/6lowpan/rx.c:329:32:    got restricted lowpan_rx_result ...

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