Bluetooth: Controller: df: Fix CTE reqest proc enable if feat supported

The CTE request procedure should be enabled only if a peer supports
CTE response feature. That information can be obtained by feature
exchange procedure. If there were no feature exchange then CTE
request feature may be disabled if a peer responses with LL_UNKNOWN_RSP
for a CTE request.

The implementation of ll_df_set_conn_cte_req_enable was checking if
CTE response feature is supported only when there was feature exchange.
There was missing possibility to stop CTE request if a peer responded
with LL_UNKNOWN_RSP for an earlier CTE request.

The commit changes the implementation of ll_df_set_conn_cte_req_enable.
The CTE response feature check is moved to ull_cp_cte_req function,
because it belongs more to control procedure than to function that
handles Host request to start the procedure.
Second change is related with use of conn->llcp.fex.features_used.
It stores information about features supported by peer. It does
not depend on execution of the feature exchange control procedure.

By the way, there were removed else statement in ll_df_set_conn_cte_-
req_enable because it was not needed.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>