Revert "brcmfmac: Disable power management"

Shortly after the release of the Pi 3B, a loss of SSH connectivity
over WiFi was traced to the power management handling, so power
management was disabled. And so it has remained ever since.

Enabling power management saves 55mA (~270mW) on a Pi 4B, so is very
much worth the minimal effort of reverting this patch, which was
squashed and rebased many times since then to the commit hash is
meaningless.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>