esp_wifi_set_channel() function

Set primary/secondary channel of device @attention 1. This API should be called after esp_wifi_start() and before esp_wifi_stop() @attention 2. When device is in STA mode, this API should not be called when STA is scanning or connecting to an external AP @attention 3. When device is in softAP mode, this API should not be called when softAP has connected to external STAs @attention 4. When device is in STA+softAP mode, this API should not be called when in the scenarios described above @attention 5. The channel info set by this API will not be stored in NVS. So If you want to remember the channel used before WiFi stop, you need to call this API again after WiFi start, or you can call `esp_wifi_set_config()` to store the channel info in NVS.