pm: device: do not call device if already on the given state

If the device is already at the given state, do not call the device PM
control function. This makes sure that devices are only called to change
from one state to another.

Even though asynchronous device PM is completely broken, transitional
states are considered too.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>