i2c and spi: clarify with SetSpeed vs LimitSpeed.

Otherwise when reading code is it very unclear if it may be slower or not.

Yet another breaking change.

experimental/devices/bitbang/i2c.go

@@ -88,8 +88,8 @@ func (i *I2C) Tx(addr uint16, w, r []byte) error {
 	return nil
 }
 
-// Speed implements i2c.Bus.
+// SetSpeed implements i2c.Bus.
-func (i *I2C) Speed(hz int64) error {
+func (i *I2C) SetSpeed(hz int64) error {
 	i.mu.Lock()
 	defer i.mu.Unlock()
 	i.halfCycle = time.Second / time.Duration(hz) / time.Duration(2)

experimental/devices/bitbang/spi.go

@@ -53,8 +53,8 @@ func (s *SPI) Duplex() conn.Duplex {
 	return conn.Full
 }
 
-// Speed implements spi.ConnCloser.
+// LimitSpeed implements spi.ConnCloser.
-func (s *SPI) Speed(maxHz int64) error {
+func (s *SPI) LimitSpeed(maxHz int64) error {
 	if maxHz <= 0 {
 		return errors.New("bitbang-spi: invalid maxHz")
 	}