ALSA: 6fire: Fix probe of multiple cards

The probe code of snd-usb-6fire driver overrides the devices[] pointer
wrongly without checking whether it's already occupied or not.  This
would screw up the device disconnection later.

Spotted by coverity CID 141423.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/6fire/chip.c

@@ -101,7 +101,7 @@ static int usb6fire_chip_probe(struct usb_interface *intf,
 			usb_set_intfdata(intf, chips[i]);
 			mutex_unlock(&register_mutex);
 			return 0;
-		} else if (regidx < 0)
+		} else if (!devices[i] && regidx < 0)
 			regidx = i;
 	}
 	if (regidx < 0) {