pm: device: simplify suspend checks

The shared _pm_devices function used should_suspend check function to
see if a device had to be suspended or not. Some of the logic inside
that function was redundant since the pm_device_state_set function
already performs similar checks, e.g. if the device is already at the
given state or the state transition is not supported it will return
error codes appropriately.

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