[DCCP] ccid3: Fix two bugs in sending rate computation

This fixes
 1) a bug in the recomputation of the sending rate by the nofeedback
    timer when no feedback at all has so far been sent by the receiver:
    min_t was used instead of max_t, which is wrong (cf. RFC 3448, p. 10);

 2) an error in the computation of larger initial windows: instead of
    min(... max()) (cf. RFC 4342, 5.), the code had used max(... max()).

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>