HID: hid-logitech-dj: querying_devices was never set

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: Nestor Lopez Casado <nlopezcasad@logitech.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>