Weirdly we have to stop the AP too (which we never started),
or cyw43_tcpip_link_status still reports that STA is connected.
As long as AP mode isn't implemented, this doesn't matter and
we can just do it.
.. the value actually needs to be enforced each time the STA or AP
is enabled, because internally there's a call to cyw43_wifi_pm with the
library's defaut power management value, not ours.
Add a getter, though it only returns our idea of what the power
management register is set to, it doesn't read out from the actual
hardware, sadly.
My pings go out, and then they come back
```py
import os
import wifi
import ipaddress
wifi.radio.connect(os.getenv('WIFI_SSID'), os.getenv('WIFI_PASSWORD'))
ipv4 = ipaddress.ip_address("8.8.4.4")
print("Ping google.com: %f ms" % (wifi.radio.ping(ipv4)*1000))
```
