Return BLE_HS_EDONE in characteristic discovery

[geovie]

If the characteristic is not the last one in a service next_char_cb will be called again with the next characteristic which in the current implementation would override the end handle and would again send the state signal.

The double state signal then leads to get_descriptors returning an empty array, can be tested with this sample code:

let characteristic = service
        .get_characteristic(characteristic_uuid)
        .await
        .unwrap();

    let uuids: Vec<_> = characteristic
        .get_descriptors()
        .await
        .unwrap()
        .map(|d| d.uuid().to_string())
        .collect();
    log::info!("subscribe to {:?}", uuids);

I haven't found any documentation about what the callback is supposed to return but I found out that returning HS_DONE would not call the callback again. Also Nimble-Arduino also does this so it seems to be correct: https://github.com/wakwak-koba/NimBLE-Arduino/blob/aec7e4294dc2ede1434b68cf8381a307f2c08c68/src/NimBLERemoteCharacteristic.cpp#L216C14-L216C26

[taks]

Thanks.