crypto: atmel-sha - fix .import()/.export() implementation

Using only the digest, digcnt[], bufcnt and buffer[] fields of the
struct atmel_sha_reqctx was not enough to import/export the request state,
so now we use the whole structure.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>