ESP32-C6: don't use LP (low-power) UART

[dhalbert]

• Fixes ESP32-C6: board.UART() on GPIO16 and GPIO17 is returning "Invalid pins" #9577.

1. ESP32-C6 and ESP32-P4 have a new "low-power" UART, which has less functionality and only works on certain pins. It is in the regular UART numbering scheme. Don't use it, at least for now (it has some wake-up ability for sleep, so it might be interesting in the long run).
2. UART() constructor was not cleaning up when uart_set_pin() failed.
3. Loop searching for a free UART was picking the last available one instead of the first available one.
4. Also removed redundant CIRCUITPY_ESP_USB_SERIAL_JTAG = 1 from a C6 board. It's already set by default. (I had removed that line while attempting to debug this.)

[tannewt]

Thanks for figuring this out!