crypto: tcrypt - fix buflen reminder calculation

- This fixes the intent of the code to limit the last scatterlist to
  either a full PAGE or a fraction of it, depending on the number of
  pages needed by buflen and the available space advertised by XBUFLEN.

  The original code always sets the last scatterlist to a fraction of a
  PAGE because the first 'if' is never executed.

- Rearrange the second part of the code to remove the conditional from
  the loop

Signed-off-by: Cristian Stoica <cristian.stoica@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>