Set querying_devices flag to true when we start the enumeration
process.

This was missing from the original patch. It never produced
undesirable effects as it is highly improbable to have a second
enumeration triggered while a first one was still in progress.
Signed-off-by: NNestor Lopez Casado <nlopezcasad@logitech.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>