Bluetooth: Use async request for LE enable/disable

This patch updates the code to use an asynchronous request for handling
the enabling and disabling of LE support. This refactoring is necessary
as a preparation for adding advertising support, since when LE is
disabled we should also disable advertising, and the cleanest way to do
this is to perform the two respective HCI commands in the same
asynchronous request.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>