Don't return success on a stub

We shouldn't return PSA_SUCCESS from a function that isn't implemented.
PSA_ERROR_NOT_SUPPORTED seems like the most appropriate return status
for a function that isn't implemented.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/tf-psa-crypto/core/psa_crypto_ecp.c b/tf-psa-crypto/core/psa_crypto_ecp.c
index 77bc0b2..749e11b 100644
--- a/tf-psa-crypto/core/psa_crypto_ecp.c
+++ b/tf-psa-crypto/core/psa_crypto_ecp.c
@@ -610,7 +610,7 @@
     (void) operation;
     (void) attributes;
 
-    return PSA_SUCCESS;
+    return PSA_ERROR_NOT_SUPPORTED;
 }
 
 psa_status_t psa_generate_key_iop_complete(
@@ -620,7 +620,7 @@
     (void) operation;
     (void) key;
 
-    return PSA_SUCCESS;
+    return PSA_ERROR_NOT_SUPPORTED;
 }
 
 psa_status_t psa_generate_key_iop_abort(
@@ -628,7 +628,7 @@
 {
     (void) operation;
 
-    return PSA_SUCCESS;
+    return PSA_ERROR_NOT_SUPPORTED;
 }
 
 /****************************************************************/