Bluetooth: Fix changing advertising setting while LE is connected

We only (re)enable advertising when LE is disconnected. Trying to enable
advertising using mgmt_set_advertising while connected should simply
change the flag but not do anything else (until the connection gets
dropped). This patch fixes this by making an LE connection lookup to
determine whether there are any connected devices or not.

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