regmap: regcache: Don't sync read-only registers

regcache_maple_sync() tries to sync all cached values no matter
whether it's writable or not.  OTOH, regache_sync_val() does care the
wrtability and returns -EIO for a read-only register.  This results in
an error message like:
  snd_hda_codec_realtek hdaudioC0D0: Unable to sync register 0x2f0009. -5
and the sync loop is aborted incompletely.

This patch adds the writable register check to regcache_sync_val() for
addressing the bug above.

Note that, although we may add the check in the caller side
(regcache_maple_sync()), here we put in regcache_sync_val(), so that a
similar case like this can be avoided in future.

Fixes: f033c26d ("regmap: Add maple tree based register cache")
Link: https://lore.kernel.org/r/877cs7g6f1.wl-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20230613112240.3361-1-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regcache.c

@@ -284,6 +284,9 @@ static bool regcache_reg_needs_sync(struct regmap *map, unsigned int reg,
 {
 	int ret;
 
+	if (!regmap_writeable(map, reg))
+		return false;
+
 	/* If we don't know the chip just got reset, then sync everything. */
 	if (!map->no_sync_defaults)
 		return true;