core/hw/rsa: Make GetSignature() a const member function

This doesn't directly modify instance state, so it can be made const.

src/core/hw/aes/key.cpp

@@ -223,7 +223,7 @@ void LoadNativeFirmKeysOld3DS() {
         return;
     }
 
-    auto rsa = RSA::GetSlot(0);
+    const auto rsa = RSA::GetSlot(0);
     if (!rsa) {
         LOG_ERROR(HW_AES, "RSA slot is missing");
         return;