ASoC: tas2783-sdw: do not cache read-only Controls

The read-only SDCA Controls are readings of device state, but the regmap
caches them and tas2783_reg_default[] gives each one a placeholder, so a
read is answered from the cache and never reaches the peripheral.  The
answer is whatever the cache happens to hold: on an ASUS ProArt PX13 a
read of the PDE23 actual power state returns the 0x3 placeholder, i.e.
PS3, while the Function is powered and a cache-bypassing read of the
same Control over the bus answers 0x0.  Once the cache has been given a
value it keeps it, so the Control reports a state the device left long
ago.

Mark those Controls volatile and drop their entries from the defaults.
regcache_read() and regcache_write() skip volatile registers, so a
read-only Control no longer enters the cache at all: reads reach the
peripheral, and a sync cannot attempt to write a reading back.  That is
what regcache_sync() asks for in the first place - "any registers that
should not be synced should be marked as volatile".

The list is the same one the writeable_reg callback needs, so move it
into a helper both callbacks use.  The File Download Status is not on
it, because the host writes that Control, but it is volatile all the
same: the device raises its own bits on its own schedule, and
sdca_fdl_process() reads the Control to see them.  A cached answer would
hide the device half of the handshake.

Nothing in the driver reads any of these Controls - it performs no
register reads at all - so the change is visible only through the
regmap.

Signed-off-by: Andrey Golovko <andrey.golovko@gmail.com>
Link: https://patch.msgid.link/20260815112000.4180-3-andrey.golovko@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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