math/mpx-mul4-*-sse2.S (mpxmont_redc4): Fix end-of-outer-loop commentary.

  * The carry loop is wrong if the destination is an exact multiple of
    four limbs.  Fortunately, it isn't.

  * The initial pass feeds into the main loop unconditionally, unlike
    `mpxmont_mul4_...' (from which I think the commentary was
    uncritically copied), so being at the end of it doesn't tell you
    anything about whether to start another.  And, indeed, we do indeed
    check the loop-end condition.