[ddk][i2c] Phase 2 of i2c protocol clean-up

- Remove i2c_channel_t. Now the i2c protocol ops use a uint32_t index
  to specify the i2c channel

- Remove i2c_set_bitrate() from the high level i2c protocol.
  None of our drivers actually implement this, and if they did
  this would be something set by the platform board driver,
  not the client.

- Remove the "actual" parameter from i2c_complete_cb.
  i2c read transfers either return the amount requested or fail,
  so this parameter is not needed.

Change-Id: I2dc2499ba1218d9bfd3501a43982279a3c534157