USB: chaoskey read offset bug

Rng reads in chaoskey driver could return the same data under
the certain conditions.

Signed-off-by: Alexander Inyukhin <shurick@sectorb.msk.ru>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>