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_encrypt.c b/programs/pkey/rsa_encrypt.c
index 4bbb54e..6538f8a 100644
--- a/programs/pkey/rsa_encrypt.c
+++ b/programs/pkey/rsa_encrypt.c
@@ -126,7 +126,7 @@
         goto exit;
     }
 
-    for (i = 0; i < rsa.MBEDTLS_PRIVATE(len); i++) {
+    for (i = 0; i < mbedtls_rsa_get_len(&rsa); i++) {
         mbedtls_fprintf(f, "%02X%s", buf[i],
                         (i + 1) % 16 == 0 ? "\r\n" : " ");
     }