Bluetooth: Limit BR/EDR switching for LE only with secure connections

When a powered on dual-mode controller has been configured to operate
as LE only with secure connections, then the BR/EDR side of things can
not be switched back on. Do reconfigure the controller it first needs
to be powered down.

The secure connections feature is implemented in the BR/EDR controller
while for LE it is implemented in the host. So explicitly forbid such
a transaction to avoid inconsistent states.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>