crypto: mv_cesa - move digest state initialisation to a better place

On one hand, the digest state registers need to be set only when
actually using the crypto engine. On the other hand, there is a check
for ctx->first_hash in mv_process_hash_current() already, so use that.

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>