regmap: regmap_raw_read return error on !bus->read

Return -ENOTSUPP if map->bus->read is not implemented and we do not use
the cache. This code path would directly use bus->read would run into an
NULL pointer for the read function.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>