Add more atomicity in usb_serial_jtag.c; handle C6 in more places

[dhalbert]

• Fixes Sending files to esp32c6 boards fails almost 100% of the time. #9399.

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-- Some conditional checking for ESP32-C6 was missing.
-- Be more careful that ringbuf use in usb_serial_jtag.c is atomic. ringbuf_get() is not atomic, for instance.

I tried just leaving the USB_SERIAL_JTAG_INTR_SERIAL_OUT_RECV_PKT interrupt enabled all the time, which would have made the code simpler, but that did not work.

This may fix some other reported flakiness, not sure.

Tested by writing larger files with Thonny to an espressif_esp32c6_devkitc_1_n8. That's the only C6 board I have right now.

[tannewt]

Great! Thank you!