regmap: Drop check whether a register is readable in regcache_read

One of the reasons for using a cache is to have a software shadow of a register
which is writable but not readable. This allows us to do a read-modify-write
operation on such a register.

Currently regcache checks whether a register is readable when performing a
cached read and returns an error if it is not. Drop this check, since it will
prevent us from using the cache for registers where read-back is not possible.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/regcache.c

@@ -199,9 +199,6 @@ int regcache_read(struct regmap *map,
 
 	BUG_ON(!map->cache_ops);
 
-	if (!regmap_readable(map, reg))
-		return -EIO;
-
 	if (!regmap_volatile(map, reg))
 		return map->cache_ops->read(map, reg, value);