Complete gpio implementation

* When changing pin direction and/or value, I'm now using
  MCP2210_CMD_SET_PIN_DIR and MCP2210_CMD_SET_PIN_VALUE (respectively)
  instead of MCP2210_CMD_SET_CHIP_CONFIG as I'm presuming that this is a
  lighter weight operation on the chip.
* The dev->s.chip_settings stays updated when these commands are sent
  now (rather from within the driver or the ioctl interface).
* Added support to retrieve values from input pins.