Bluetooth: Restrict disabling of HS when controller is powered off

Disabling the high speed setting when the controller is powered on has
too many side effects that are not taken care of. And in general it
is not an useful operation anyway. So just make such a command fail
with a rejection error message.

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