programs_rsa_encrypt/decrypt: Updated programs to use `mbedtls_rsa_get_len()`

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/programs/pkey/rsa_decrypt.c b/programs/pkey/rsa_decrypt.c
index 76bfddf..a84af50 100644
--- a/programs/pkey/rsa_decrypt.c
+++ b/programs/pkey/rsa_decrypt.c
@@ -133,7 +133,7 @@
 
     fclose(f);
 
-    if (i != rsa.MBEDTLS_PRIVATE(len)) {
+    if (i != mbedtls_rsa_get_len(&rsa)) {
         mbedtls_printf("\n  ! Invalid RSA signature format\n\n");
         goto exit;
     }