net: lwm2m: lwm2m_message_handling: fix validation of time.

Currently the if statement checks if the write_buf_len is the
sizeof(time_t), when CONFIG_LWM2M_ENGINE_VALIDATION_BUFFER_SIZE
is > 0 it will always assume the the resource buf len is
not supported unless CONFIG_LWM2M_ENGINE_VALIDATION_BUFFER_SIZE
is equal to sizeof(time_t). To fix this the check should be
CONFIG_LWM2M_ENGINE_VALIDATION_BUFFER_SIZE >= sizeof(time_t).

Signed-off-by: Brandon Allen <brandon.allen@exacttechnology.com>

subsys/net/lib/lwm2m/lwm2m_message_handling.c

@@ -1160,7 +1160,7 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst, struct lwm2m_eng
 				break;
 			}
 
-			if (write_buf_len == sizeof(time_t)) {
+			if (write_buf_len >= sizeof(time_t)) {
 				*(time_t *)write_buf = temp_time;
 				len = sizeof(time_t);
 			} else if (write_buf_len == sizeof(uint32_t)) {