crypto: api - Fix aligned ctx helper

The aligned ctx helper was using a bogus alignment value thas was
one off the correct value.  Fortunately the current users do not
require anything beyond the natural alignment of the platform so
this hasn't caused a problem.

This patch fixes that and also removes the unnecessary minimum
check since if the alignment is less than the natural alignment
then the subsequent ALIGN operation should be a noop.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>