[usb] Use correct length for memcpy()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/bus/usb.c

 		}
 
 		/* Describe function */
-		memcpy ( &desc->class, &association->class,
-			 sizeof ( desc->class ) );
+		memcpy ( &desc->class.class, &association->class,
+			 sizeof ( desc->class.class ) );
 		desc->count = association->count;
 		for ( i = 0 ; i < association->count ; i++ )
 			interfaces[i] = ( first + i );