Bluetooth: Read LE Advertising Channel TX Power only when available

The Read LE Advertising Channel TX Power command is not mandatory for
a Bluetooth HCI controller only supporting receiption. Move the command
to the third stage of the controller initialization and only execute it
when support for it has been indicated.

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