pm: policy: return a reference to the next state

Return a constant reference to the next state instead of a copy of
struct pm_state_info. When the next state should be active, just return
NULL. Struct copying should be in general avoided, specially in code
paths executed frequently as is this one.

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