ASoC: JZ4770: codec: Removed bypass_to_hp and bypass_to_lineout

These flags were never set.
Also removed lineout_enable, all uses of which were commented out.
ASoC: JZ4770: codec: Separate DAC start/stop sequence from playback

Instead, put the mute code in the digital_mute handler and put the
code that forces the sample value to zero in an event handler for the
DAC DAPM widget.

Also removed a few delays that didn't seem to serve any purpose.
ASoC: JZ4770: codec: Removed debug parameter

I find it easier to add debug prints at the moment I decide I want to
have a certain piece of information logged.
ASoC: JZ4770: codec: Removed code that forces the sample value to zero

The comment says it is an anti-pop workaround, but I hear a slight pop
both with and without this code. Besides, code that puts samples into
the transmit buffer belongs in the jz4770-i2s driver, not in the codec
driver.
ASoC: JZ4770: codec: Force DAC to be enabled during playback

In commit e415baca I wrote that there is no need to keep the DAC active
when all outputs are disabled, but that turns out to be false: if the
DAC is powered down, the codec will no longer consume samples from the
AIC and as a result user space can block on the delivery of samples.

I solved this by making the SYSCLK output explicit and forcing this
output to be enabled during playback, which in turn keeps the DAC
powered.

The capture stream might require a similar construct, but for the
moment I'm only interested in playback.

Thanks to Lars-Peter Clausen for his help in figuring out the problem.
ASoC: JZ4770: GCW0: Reduce pop on HP amp power up

The 50 ms delay was determined experimentally: I couldn't hear a
difference when increasing the the delay, while the pop when using
a delay of 10 ms was slightly louder.