HID: hid-steam: Make client_opened a counter

The client_opened variable was used to track if the hidraw was opened by any
clients to silence keyboard/mouse events while opened. However, there was no
counting of how many clients were opened, so opening two at the same time and
then closing one would fool the driver into thinking it had no remaining opened
clients.

Signed-off-by: Vicki Pfau <vi@endrift.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>