Bluetooth: conn: Don't deadlock on `BT_RECV_WORKQ_SYS`

My recent change overlooked this configuration, which is not tested in CI
currently.

If someone had enabled it, they would've just gotten an unfixable assert.

The correct test would be checking if the current thread is the system wq,
but I think the compiler can't know that and can't remove the "dead" code.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>