[DCCP]: Make code assumptions explicit

[DCCP]: Make code assumptions explicit

This removes several `XXX' references which indicate a missing support
for non-1-byte feature values: this is unnecessary, as all currently known
(standardised) SP feature values are 1-byte quantities.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>