Fix bootloader not replying to SAM-BA commands if USB Reset happens

[umaplehurst]

Steps to reproduce:

a) power up the board cold from a separate power supply, b) plug in board to PC with USB cable, run bossac to say to hello to the Arduino; all good so far; c) now unplug and replug the USB cable, while keeping the board powered, d) notice that USB re-enumeration takes place OK, the device is still detected just fine by the PC; e) however, if you now run bossac, it just hangs and gets no reply;

This is because the read_job flag does not get reset back to false when a USB Reset event occurs.

[CLAassistant]

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