net: lwm2m: Fix pmin handling on tickless

If observed resource was written during the pMin period, it did
not schedule any wake-up event into the future. Notify message would
then only be generated as a result of any other (like Update) event.

Refactor check_notifications() to follow same pattern as retransmit_req().
Return the next event timestamp, which could be now.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>