sane-desc: fix udev hwdb generation

Generated hwdb files listed the vendor ID instead of the product ID
which made udevd not recognizing scanner devices as such. Thanks to
Fabrice Bellet who spotted the problem.

tools/sane-desc.c

@@ -3827,7 +3827,7 @@ print_hwdb (void)
 
       for(j = 0; j < 4; j++) {
         vendor_id[j] = toupper(vendor_id[j]);
-        product_id[j] = toupper(vendor_id[j]);
+        product_id[j] = toupper(product_id[j]);
       }
 
       printf ("usb:v%sp%s*\n libsane_matched=yes\n\n",