DPP: Clear dpp_listen_freq on remain-on-channel failure

DPP: Clear dpp_listen_freq on remain-on-channel failure

If the DPP_LISTEN command failed due to the driver rejecting the
remain-on-channel request, wpa_s->dpp_listen_freq was left set to the
requested listen frequency and this resulted in the next DPP_LISTEN for
the same frequency reporting "DPP: Already listening on .." even when
the driver was not really listening on that frequency. Fix this by
clearing wpa_s->dpp_listen_freq in the error case.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>