fix BLE write limits

[positron96]

Fixes #57

Fixes 2 issues:

• WebBluetooth transport had tx_limit which were not used in any way, parent Transport used its own writeChunk. Now WebBluetooth transport updates parent field, which causes transmissions to split data properly.
• For Nordic UART Service, tx_limit was 241, when all of Nordic Semi builds of micropython have max characteristic size of 20. It's possible that other builds (e.g. ESP32) have a larger characteristic, but it can't be relied upon anyway. It's set to 20 now.