device-purism-librem5: Disable modem power management

According to https://source.puri.sm/Librem5/linux/-/issues/303#note_160864 enabling the modem power management results in a unreliable modem, but more power savings.

With power management on the modem can disconnect at any time, including during calls. This makes the Librem 5 a very bad phone. Instead let's ensure it works as a phone with a semi-reliable modem (there is still the sleep/resume issue https://source.puri.sm/Librem5/linux/-/issues/423) at the cost of battery life.

As the Librem5's battery life is already poor, a little bit worse isn't much different. Either way you need an external battery pack.