crypto: img-hash - Fix hash request context

Move 0 length buffer to end of structure to stop overwriting
fallback request data. This doesn't cause a bug itself as the
buffer is never used alongside the fallback but should be
changed.

Signed-off-by: Will Thomas <will.thomas@imgtec.com>
Reviewed-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>