hal_uart.h文件中只有下面几种波特率，我想设置为4800，如何设置？
/* UART Ports */
/*
Serial Port Baudrate Settings
Have to match with baudrate table
*/
#define HAL_UART_BR_9600 0x00
#define HAL_UART_BR_19200 0x01
#define HAL_UART_BR_38400 0x02
#define HAL_UART_BR_57600 0x03
#define HAL_UART_BR_115200 0x04
