hci: contains several corrections needed for HCI on both ninafw and cyw43439

[deadprogram]

This PR contains several corrections needed for HCI to work correctly on both ninafw and cyw43439:

• separate read and write buffers to accomodate read buffer with pending partial data to be handled
• only read when there is data pending to avoid putting cwy43439 into forever loop
• handle any data in buffer from read that contains data from more than 1 packet
• skip past data for sync packets since we do not handle them anyhow
• some better naming and comments to hopefully clarify a bit what is going on

Addresses #300

[jsoriano]

Thanks! Examples work on the Nano 33 IoT with these changes.

[colececil]

Thanks so much for this fix! I was unable to enable the bluetooth adapter on my Arduino Nano 33 IoT. But after updating to your commit in my go.mod, it works! Now I'm unblocked!

[deadprogram]

Thanks for review/testing @jsoriano and @colececil now merging.