bluetooth: l2cap: fix ecred conn response for all connections refused

If all connections were refused, we haven't reached part of
le_ecred_conn_req with connecting channels - thus i was never
incremented. PDU shall be created always with the length containing
full size of scid array - we always respond with all of the CIDs filled,
they just will be all zeros when all connections were refused.

This was affecting L2CAP/ECFC/BV-26-C

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>