USB: rio500: refuse more than one device at a time

This driver is using a global variable. It cannot handle more than
one device at a time. The issue has been existing since the dawn
of the driver.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by:  <syzbot+35f04d136fc975a70da4@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>