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>