Bluetooth: Fix sending write_le_host_supporte for LE-only controllers

The Bluetooth Core Specification (4.0) defines the Write LE Host
Supported HCI command as only available for controllers supporting
BR/EDR. This is further reflected in the Read Local Extended Features
HCI command also not being available for LE-only controllers. In other
words, host-side LE support is implicit for single-mode LE controllers
and doesn't have explicit HCI-level enablement.

This patch ensures that the LE setting is always exposed as enabled
through mgmt and returns a "rejected" response if user space tries to
toggle the setting. The patch also ensures that Write LE Host Supported
is never sent for LE-only controllers.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>