If uart_driver_install is not called before uart_get_baudrate,
the UART port may not be ready. In such cases, the register values
cannot be read out, leading to a division by zero error in uart_ll_get_baudrate.
This commit adds a check to ensure the UART port is enabled before
attempting to read its registers.
Closes https://github.com/espressif/esp-idf/issues/17513
Increase LP_UART_EMPTY_THRESH_DEFAULT value to 4. The original value
could cause the FIFO become empty before filling next data into the FIFO
when the buadrate is high. TX_DONE interrupt would raise before actual
transmission complete in such case.
Song Ruo Jing
C.S.M
Linda
laokaiyao