purism-librem5: disable power management for Sparklan/brcmfmac wifi

This wifi module doesn't ship with the L5, but I have one because it's generally more stable than the redpine thing that does ship with the L5... however there's a known problem with having PM enabled on it.

This adds a NM config option, that matches specifically on this device/driver, to disable wifi. PM for the redpine should not be affected by this config change.