NimBLEClient not processing passkey correctly

[projecta98]

It appears that NimBLEClient does not process the passkey correctly for the client callback cases.

           ```
          // Compatibility only - Do not use, should be removed the in future
            if(NimBLEDevice::m_securityCallbacks != nullptr) {
                pkey.passkey = NimBLEDevice::m_securityCallbacks->onPassKeyRequest();
            /////////////////////////////////////////////
            } else {
                client->m_pClientCallbacks->onPassKeyRequest(); // <-- bug here , passkey not assigned to anything
            }

The bold line above should be: pkey.passkey = client->m_pClientCallbacks->onPassKeyRequest();

Actually this [revision ](https://github.com/h2zero/NimBLE-Arduino/commit/1aeb2fed80f64a430d26f1537b9f5811fe33a294#diff-8f692c893f6a420d294d6426dcac9a9c) has a better implementation, which for some reason was removed from the master.

My apologies if I am completely off base!

[h2zero]

Thanks for reporting this!

Looks like it's been that way for a long time, introduced this bug when I implemented default callback methods.

Pushing the fix right away.