[DCCP] ccid3: Fix warning message about illegal ACK

This avoids a (harmless) warning message being printed at the DCCP server
(the receiver of a DCCP half connection).

Incoming packets are both directed to

 * ccid_hc_rx_packet_recv() for the server half
 * ccid_hc_tx_packet_recv() for the client half

The message gets printed since on a server the client half is currently not
sending data packets.
This is resolved for the moment by checking the DCCP-role first. In future
times (bidirectional DCCP connections), this test may have to be more
sophisticated.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>