rsa: update return values of priv/pub parse/write functions

The goal is to remove usage of PK return values in order to
completely eliminate that dependency.
This commit also updates pkparse and test_suite_x509parse to
align with this change in return values.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/pkparse.c b/library/pkparse.c
index 2708c8c..17df101 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -1556,8 +1556,7 @@
         return ret;
     }
     mbedtls_pk_free(ctx);
-    if (ret != (MBEDTLS_ERROR_ADD(MBEDTLS_ERR_PK_INVALID_PUBKEY,
-                                  MBEDTLS_ERR_ASN1_UNEXPECTED_TAG))) {
+    if (ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG) {
         return ret;
     }
 #endif /* MBEDTLS_RSA_C */