ASoC: rt5670: Keep sysclk on if JD func is used

System clock is necessary for rt5670 JD function. We assume system
clock source will be set in machine driver. So there are two things
left we should do in codec driver.
1. Set sysclk to codec internal clock in probe since machine driver
   may not do that before JD function is registered.
2. Power up PLL once sysclk source is switched to PLL.

Signed-off-by: Bard Liao <bardliao@realtek.com>