crypto: caam - Clear some memory in instantiate_rng

According to the comment at the end of the 'for' loop just a few lines
below, it looks needed to clear 'desc'.

So it should also be cleared for the first iteration.

Move the memset() to the beginning of the loop to be safe.

Fixes: 281922a1

 ("crypto: caam - add support for SEC v5.x RNG4")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Gaurav Jain <gaurav.jain@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>