The connection subrating needs CONN_SUBRATING_HOST feature

Hi!
I have tried the connection subrating in the Spec 5.3 and may found an issue.
Before the central sends the `SUBRATE_IND`, it will check wether the peripheral has the `BLE_LL_FEAT_CONN_SUBRATING_HOST`. The corresponding code is in `ble_ll.c` Line 4131. However, the peripheral will not initiate this feature, so the `ble_ll_conn_subrate_req_hci()` always return `-ENOTSUP`.
A quick fix is modify the function `ble_ll_init()` at `ble_ll.c` Line 1965 to:
```C
#if MYNEWT_VAL(BLE_LL_CFG_FEAT_LL_ENHANCED_CONN_UPDATE)
features |= BLE_LL_FEAT_CONN_SUBRATING;
features |= BLE_LL_FEAT_CONN_SUBRATING_HOST;
#endif
```
By the way, it seems that there is no connection subrating API in the GAP layer, is this normal?