Bluetooth: Add a new mgmt_set_bredr command

This patch introduces a new mgmt command for enabling/disabling BR/EDR
functionality. This can be convenient when one wants to make a dual-mode
controller behave like a single-mode one. The command is only available
for dual-mode controllers and requires that LE is enabled before using
it. The BR/EDR setting can be enabled at any point, however disabling it
requires the controller to be powered off (otherwise a "rejected"
response will be sent).

Disabling the BR/EDR setting will automatically disable all other BR/EDR
related settings.

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