dccp ccid-2: Remove CCID naming redundancy 1/2

This removes a redundancy in the CCID half-connection (hc) naming scheme:
 * instead of 'hctx->tx_...', write 'hc->tx_...';
 * instead of 'hcrx->rx_...', write 'hc->rx_...';

which works because the 'type' of the half-connection is encoded in the
'rx_' / 'tx_' prefixes.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>