Change error from unknown oid to invalid name

Since the implementation no longer needs to know the oid, it makes more
sense for the error message to be an invalid name.

Signed-off-by: Agathiyan Bragadeesh <agathiyan.bragadeesh2@arm.com>
diff --git a/library/x509_create.c b/library/x509_create.c
index 6483b39..99115c4 100644
--- a/library/x509_create.c
+++ b/library/x509_create.c
@@ -274,7 +274,7 @@
         if (in_attr_type && *c == '=') {
             if ((attr_descr = x509_attr_descr_from_name(s, c - s)) == NULL) {
                 if ((oid = x509_oid_from_numericoid(s, c - s)) == NULL) {
-                    return MBEDTLS_ERR_X509_UNKNOWN_OID;
+                    return MBEDTLS_ERR_X509_INVALID_NAME;
                 } else {
                     numericoid = 1;
                 }