i2c, pin, spi: Figure out a way for driver to communicate real chosen speed back to user

[maruel]

Use cases:

• spi.Port.Connect()
• spi.PortCloser.LimitSpeed()
• i2c.Bus.SetSpeed()
• pin.PinOut.PWM()

In these cases, if the host driver had to choose a different frequency, it has no way to communicate this information back to the user.