power: clarify notion of action callback function

power: clarify notion of action callback function

The callback, action_for_state(), now denotes jobs that are necessary
before the target state, that is, it is kind of to-do list for entering
the state.

To this end, the name has been slightnly changed from action_on_state(),
which implies it is invoked right 'after' the state change, to
action_for_state(), which emphasizes it is invoked 'before' the state.

In addition, the position where the variable 'current' is updated has
been relocated to fit into the concept of action_for_state(). That is,
it is updated after the callback.

Change-Id: I47698ce9147c155c0d5e30feed0a60d65d69031b
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>