regulator: Reverse the disable sequence in regulator_bulk_disable()

Often there is a need for disabling a set of regulators in order opposite
to the enable order. Currently the function regulator_bulk_disable() walks
list of regulators in same order as regulator_bulk_enable(). This may cause
trouble, especially for devices with mixed analogue and digital circuits.
So reverse the disabling sequence of regulator_bulk_disable().
While at it, also correct the comment.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>