When restoring registers during runtime resume, we must not write to
I2S_TXDR which is the transmit FIFO as this queues up a sample to be
output and pushes all of the output channels down by one.

This can be demonstrated with the speaker-test utility:

	for i in a b c; do speaker-test -c 2 -s 1; done

which should play a test through the left speaker three times but if the
I2S hardware starts runtime suspended the first sample will be played
through the right speaker.

Fix this by marking I2S_TXDR as volatile (which also requires marking it
as readble, even though it technically isn't).  This seems to be the
most robust fix, the alternative of giving I2S_TXDR a default value is
more fragile since it does not prevent regcache writing to the register
in all circumstances.

While here, also fix the configuration of I2S_RXDR and I2S_FIFOLR; these
are not writable so they do not suffer from the same problem as I2S_TXDR
but reading from I2S_RXDR does suffer from a similar problem.

Fixes: f0447f6c ("ASoC: rockchip: i2s: restore register during runtime_suspend/resume cycle", 2016-09-07)
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org