regulator: core: Ensure we lock all regulators

The latest workaround for the lockdep interface's not using the second
argument of mutex_lock_nested() changed the loop missed locking the last
regulator due to a thinko with the loop termination condition exiting
one regulator too soon.

Reported-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>