Bluetooth: Introduce driver specific post init callback

Some drivers might have to restore certain settings after the init
procedure has been completed. This driver callback allows them to hook
into that stage. This callback is run just before the controller is
declared as powered up.

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