data-device: send INVALID_FINISH when operation != dnd

The documentation of wl_data_offer::finish states that it should be
used to signify that a drag and drop operation is completed. So send
WL_DATA_OFFER_ERROR_INVALID_FINISH when the client calls the finish
request but the operation isn't dnd.

Signed-off-by: Harish Krupo <harishkrupo@gmail.com>
(cherry picked from commit 737ac0d4)