crypto: sun4i-ss - do not dynamically set parts of the last buffer to 0

Parts of the bf buffer were dynamically set to 0. Change this to set the
whole buffer to 0 by default to avoid any mistake.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>