Bluetooth: LL/PAC/SLA/BV-01-C fails if Slave-initiated Feature Exchange is disabled

[wopu-ot]

Describe the bug
Bluetooth qualification test LL/PAC/SLA/BV-01-C fails if Slave-initiated Features Exchange is disabled (CONFIG_BT_CTLR_SLAVE_FEAT_REQ=n). This happens because the slave device with that configuration does not respond correctly to an unexpected LL_FEATURE_RSP.

To Reproduce
Run BLE qualification test LL/PAC/SLA/BV-01-C on a device configure with CONFIG_BT_CTLR_SLAVE_FEAT_REQ=n and Slave-initiated Features Exchange disabled in the ICS.

Expected behavior
All qualification tests pass

Impact
Cannot qualify BLE stack with Slave-initiated Features Exchange disabled. Enabling this entails also enabling Connection Parameter Request Procedure (these are for some reason tied together when selecting features), which is highly undesirable in our use case.

Environment (please complete the following information):

• Zephyr 2.2

Additional context
A possible fix should ideally be back-ported to the 2.2 branch