atsamd5x: fix BAUD value

src/machine/machine_atsamd51.go

@@ -1417,6 +1417,9 @@ func (spi SPI) Configure(config SPIConfig) error {
 
 	// Set synch speed for SPI
 	baudRate := SERCOM_FREQ_REF / (2 * config.Frequency)
+	if baudRate > 0 {
+		baudRate--
+	}
 	spi.Bus.BAUD.Set(uint8(baudRate))
 
 	// Enable SPI port.