all: do not use machine.TWI_FREQ_* constants

These constants were always a bit weird:

  * they use an uncommon name (TWI instead of I2C)
  * they don't follow Go naming conventions (CamelCase)
  * the constants don't provide much value: their name is their value
    (100KHZ => 100e3).

Therefore, I propose we remove them and replace them with regular
numeric constants.

examples/adt7410/main.go

@@ -15,7 +15,7 @@ var (
 
 func main() {
 
-	i2c.Configure(machine.I2CConfig{Frequency: machine.TWI_FREQ_400KHZ})
+	i2c.Configure(machine.I2CConfig{Frequency: 400e3})
 	sensor.Configure()
 
 	for {