First 64 incoming serial bytes lost after power cycle of Micro:bit

After the power cycle, the first 64 incoming serial bytes get lost. serialRead replied nothing. I've confirmed with a USB analyzer, The second byte of the response to the command 0x83 is always zero with a meanless payload. After the first 64 bytes, things get normal.

Here is how I trigger this bug in Micro:bit:

Using Interface Version: 0253 and hex file https://github.com/microbit-foundation/microbit-firmata/blob/master/precompiled/microbit-firmata-v1.0.hex

Open https://deqingsun.github.io/webUSBTest/stuff/testMicrobitDataBug/web.html in Chrome. Replug Micro:bit. Click "connect", and then click "requestFirmataVersion". The webpage will send ```F9 00 00``` and nRF51 chip replies ```F9 02 06```. The response can be captured on TP15, TGT_TX. But DAPJS can not read anything until the 22nd time you click, and the last 2 bytes are returned.


Reproduce the failure with the linked web.html in Chrome using the specified micro:bit firmware, then trace the serialRead path in dapjs while issuing the 0x83 request. Done means the response is readable immediately after reconnecting and the first 64 incoming bytes are no longer lost.

Written by the indexing model from the issue text.