regmap: Return a sensible error code if we fail to read the cache

If a register isn't cached then let callers know that so they can fall
back or error handle appropriately.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
diff --git a/drivers/base/regmap/regcache-indexed.c b/drivers/base/regmap/regcache-indexed.c
index 2e10bb1..507731a 100644
--- a/drivers/base/regmap/regcache-indexed.c
+++ b/drivers/base/regmap/regcache-indexed.c
@@ -20,11 +20,10 @@
 	int ret;
 
 	ret = regcache_lookup_reg(map, reg);
-	if (ret < 0)
-		*value = 0;
-	else
+	if (ret >= 0)
 		*value = map->reg_defaults[ret].def;
-	return 0;
+
+	return ret;
 }
 
 static int regcache_indexed_write(struct regmap *map, unsigned int reg,