p automatically when time out. Unit: us. Range: 10000 ~ 268435455(0xFFFFFFF).<ul>
<li>If sleep_time_in_us is 0xFFFFFFF, the ESP8266 will sleep till</li>
<li>if wifi_fpm_set_sleep_type is set to be LIGHT_SLEEP_T, ESP8266 can wake up by GPIO.</li>
<li>if wifi_fpm_set_sleep_type is set to be MODEM_SLEEP_T, ESP8266 can wake up by wifi_fpm_do_wakeup.</li>
</ul>
</td></tr>
  </table>
  </dd>
</dl>
<dl class="section return"><dt>Returns</dt><dd>0, setting succeed; </dd>
<dd>
-1, fail to sleep, sleep status error; 