SN32: serial: only calculate FD if required

High precision is not always required.
Since the chip doesn't have an FPU,
calculating the FD precisely is expensive on resources.
Make FD usage optional

os/hal/ports/SN32/LLD/SN32F2xx/UART/hal_serial_lld.c

@@ -68,6 +68,7 @@ static const SerialConfig default_config = {SERIAL_DEFAULT_BITRATE,
 /*===========================================================================*/
 /* Driver local functions.                                                   */
 /*===========================================================================*/
+#if SN32_UART_USE_FD || defined(__DOXYGEN__)
 void UART_divisor_CAL(uint32_t baudrate,uint32_t UART_PCLK,uint8_t Oversampling,uint8_t *dlm,uint8_t *dll,uint8_t  *d_divaddval,uint8_t  *d_mulval)
 {
   float expected_val;
@@ -156,6 +157,8 @@ void UART_divisor_CAL(uint32_t baudrate,uint32_t UART_PCLK,uint8_t Oversampling,
     *d_divaddval = divaddval[0];
   }
 }
+#endif
+
 /**
  * @brief   UART initialization.
  * @details This function must be invoked with interrupts disabled.
@@ -173,7 +176,15 @@ static void uart_init(SerialDriver *sdp, const SerialConfig *config) {
   apbclock = (SN32_HCLK);
 
   // Calculate divider
+#if SN32_UART_USE_FD || defined(__DOXYGEN__)
   UART_divisor_CAL(config->speed,apbclock,oversampling,&dlm,&dll,&divaddval,&mulval);
+#else
+  uint32_t divisor = (uint32_t)(apbclock/oversampling/config->speed);
+  dlm = (divisor>>8)&0xff;
+  dll = divisor&0xff;
+  divaddval = 0;
+  mulval = 1;
+#endif // SN32_UART_USE_FD
 
   // Update the registers
   u->LC = UART_Divisor_Latch_Access_Enable;