ASoC: nau8540: fix the record pop noise

When the record starts, the driver turns on MICBIAS and the voltage is
pulled up for an instant. If the receiver starts to capture the signal
between the instant, there is an pop noise in the stream beginning.
To avoid the pop noise, the driver makes a delay in the sequence.
After MICBIAS powered up, the driver waits 300 ms for the voltage
going down. Then turns on the ADC output, and sends signal to receiver.
The pop noise can be erased.

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