ASoC: nau8824: recover system clock when device changes

User reports an issue in Ubuntu about the device switch upon playback.
We find the FLL will disalbe when switching headphone to speaker.
The pulseaudio will stop the headphone and close its power. Then,
it just opens the speaker and turn on its power. Therefore,
the supply of system clock does the OFF event and disables FLL.
But the FLL doesn't enable again when the speaker powers on.

The patch adds the recovery of system clock to enable FLL again
for this case. And it covers the case that system clock from MCLK.

Signed-off-by: John Hsu <KCHSU0@nuvoton.com>
Signed-off-by: Mark Brown <broonie@kernel.org>