crypto: ecdsa - Use ecc_digits_from_bytes to create hash digits array

Since ecc_digits_from_bytes will provide zeros when an insufficient number
of bytes are passed in the input byte array, use it to create the hash
digits directly from the input byte array. This avoids going through an
intermediate byte array (rawhash) that has the first few bytes filled with
zeros.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>