fix(uart): clock source (espressif#10172)

Problem detected with ESP32 and ESP32-S2 when the baudrate goes to 460600 bps.

REF_TICK (2MHz) seem not to work properly.
Limiting the use of REF_TICK for up to 205 Kbps.

Author: SuGlider

cores/esp32/esp32-hal-uart.c

@@ -508,8 +508,8 @@ uart_t *uartBegin(
 #if SOC_UART_SUPPORT_XTAL_CLK
   uart_config.source_clk = UART_SCLK_XTAL;  // valid for C2, S3, C3, C6, H2 and P4
 #elif SOC_UART_SUPPORT_REF_TICK
-  if (baudrate <= 1000000) {
-    uart_config.source_clk = UART_SCLK_REF_TICK;  // valid for ESP32, S2 - MAX supported baud rate is 1MHz
+  if (baudrate <= 250000) {
+    uart_config.source_clk = UART_SCLK_REF_TICK;  // valid for ESP32, S2 - MAX supported baud rate is 250 Kbps
   } else {
     uart_config.source_clk = UART_SCLK_APB;  // baudrate may change with the APB Frequency!
   }