staging: wfx: improve error handling of hif_join()

Smatch complains:

   hif_tx.c:319 hif_join() error: we previously assumed 'channel' could be null (see line 315)
   311          if (!hif)
   312                  return -ENOMEM;
   313          body->infrastructure_bss_mode = !conf->ibss_joined;
   314          body->short_preamble = conf->use_short_preamble;
   315          if (channel && channel->flags & IEEE80211_CHAN_NO_IR)
                    ^^^^^^^
   316                  body->probe_for_join = 0;
   317          else
   318                  body->probe_for_join = 1;
   319          body->channel_number = channel->hw_value;
                                       ^^^^^^^^^^^^^^^^^
   320          body->beacon_interval = cpu_to_le32(conf->beacon_int);
   321          body->basic_rate_set =

Indeed, channel can't be NULL (else I would have seen plenty of Ooops
this past year). This patch explicitly claims this restriction.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201009171307.864608-2-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/hif_tx.c

@@ -308,11 +308,13 @@ int hif_join(struct wfx_vif *wvif, const struct ieee80211_bss_conf *conf,
 	WARN_ON(!conf->basic_rates);
 	WARN_ON(sizeof(body->ssid) < ssidlen);
 	WARN(!conf->ibss_joined && !ssidlen, "joining an unknown BSS");
+	if (WARN_ON(!channel))
+		return -EINVAL;
 	if (!hif)
 		return -ENOMEM;
 	body->infrastructure_bss_mode = !conf->ibss_joined;
 	body->short_preamble = conf->use_short_preamble;
-	if (channel && channel->flags & IEEE80211_CHAN_NO_IR)
+	if (channel->flags & IEEE80211_CHAN_NO_IR)
 		body->probe_for_join = 0;
 	else
 		body->probe_for_join = 1;