net: lwm2m: fix periodic services handling

This fixes an issue where if timestamp == service_due_timestamp,
we don't call the periodic service.  Then the following call to
engine_next_service_timeout_ms() returns 0 because the service
is still due and lwm2m_engine_service() is called again.
This process repeats several times until the value of
k_uptime_get() changes and then the work is finally handled.

Previously, the resolution of k_uptime_get() was in ms.  A recent
change to this API defaults Zephyr so that the resolution is
set via CONFIG_SYS_CLOCK_TICKS_PER_SEC (default 100).

This means the value of k_uptime_get() only changes every 10ms.

Reported-by: Github User pieterjanc
Signed-off-by: Michael Scott <mike@foundries.io>