regmap: Allow regmap_bulk_read() to work for "no-bus" regmaps

regmap_bulk_read() should decay to performing individual reads if
we're using a "no-bus" regmap. Unfortunately, it returns an
error because there is no map->bus pointer. Fix it.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@linaro.org>