cp2112: a number of cleanups and improvements

• hoist all request / response structures from function level to top level
• replace magic numeric literals with constants
• regroup types, data and functions
• remove setting of the id field in responses as they are completely overwritten with data from the device
• centralize setting of the id field as it is always set to the value of request type
• fix setting and querying of open-drain vs push-pull configuration of an output pin -- it's always in one of those configurations
• detect special pin configurations: a pin in a special configuration is neither general purpose input or output
• there is still no support for setting special configurations

MFC after: 2 weeks