[DCCP]: Fix elapsed time option as per section 13.2 of spec v11

The elapsed time can be two bytes or four bytes only.

Signed-off-by: Ian McDonald <iam4@cs.waikato.ac.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>