Stop marking default digest for EC keys as mandatory

ASN1_PKEY_CTRL_DEFAULT_MD_NID is documented to return 2 for a mandatory
digest algorithm, when the key can't support any others. That isn't true
here, so return 1 instead.

Partially fixes #7348

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>

(cherry picked from commit eb7eb1378c)

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7609)

crypto/ec/ec_ameth.c

@@ -505,7 +505,7 @@ static int ec_pkey_ctrl(EVP_PKEY *pkey, int op, long arg1, void *arg2)
 
     case ASN1_PKEY_CTRL_DEFAULT_MD_NID:
         *(int *)arg2 = NID_sha256;
-        return 2;
+        return 1;
 
     case ASN1_PKEY_CTRL_SET1_TLS_ENCPT:
         return EC_KEY_oct2key(EVP_PKEY_get0_EC_KEY(pkey), arg2, arg1, NULL);