HID: usbhid: no flushing if device is already polled

During open() it is unnecessary to wait for the device to flush
stale inputs if the device is polled while closed due to a quirk
or opening fails.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>