net: lwm2m: Fix IPSO Push Button counter incrementation

The Counter resource in the IPSO Push Button object was incremented
silently by the post write handler of the State resource. This prevented
the resource from being marked as updated, effectively preventing
the engine from sending notifications to observers.

Fix this, by setting the new counter value with `lwm2m_engine_set_u64()`
instead. This will update the resource value, and trigger the engine to
send notifications if needed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>