crypto: pcrypt - Dont calulate a callback cpu on empty callback cpumask

If the callback cpumask is empty, we crash with a division by zero
when we try to calculate a callback cpu. So we don't update the callback
cpu in pcrypt_do_parallel if the callback cpumask is empty.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>