Bluetooth: Host: do not overwrite result in le_ecred_conn_req

For more than one requested channels in le_ecred_conn_req only result is
overwritten in every iteration. This causes an issues if after failed
l2cap_chan_accept occures successful one: returned result will be
BT_L2CAP_LE_SUCCESS, where in reality should be returned error
and non-zero Destination CIDs will signal "Some connections refused".
Now, overwrite error only if the last one failed. If no errors occur,
result will remain BT_L2CAP_LE_SUCCESS.

This was affecting test cases L2CAP/ECFC/BV-20-C

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