regulators: helpers: Fix handling of bypass_val_on in get_bypass_regmap

The handling of bypass_val_on that was added in
regulator_get_bypass_regmap is done unconditionally however
several drivers don't define a value for bypass_val_on. This
results in those drivers reporting bypass being enabled when
it is not. In regulator_set_bypass_regmap we use bypass_mask
if bypass_val_on is zero. This patch adds similar handling in
regulator_get_bypass_regmap.

Fixes: commit dd1a571d ("regulator: helpers: Ensure bypass register field matches ON value")
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>