Change default baud rate back to 74906bps, compatible with boot ROM rate

This reverts a behaviour change from 3ceadfc0a6.

examples/serial_echo/serial_echo.c

@@ -10,6 +10,7 @@
 
 void user_init(void)
 {
+    uart_set_baud(0, 115200);
     printf("SDK version:%s\n", sdk_system_get_sdk_version());
     printf("Going into echo mode...\n");