net: lwm2m: Allow stopping while paused

When LwM2M engine is paused, stop functionality just
marks the intent and when we resume, it will send the
de-registration if that was requested.
Otherwise it will stay in SUSPENDED state until resumed
and then goes into ENGINE_DEREGISTERED -> IDLE.

This also removes the blocking loop from lwm2m_rd_client_stop().
It does not need to block because lwm2m_rd_client_start()
is already checking if client is in IDLE.

Fixes #56254

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