hwmon: (mcp3021) Prepare MCP3021 driver to support other chips

This Patch is to prepare the MCP3021 driver to support
other chips like the MCP3221. The hard defined chip data
is now stored within the data struct of each chip.

Signed-off-by: Sven Schuchmann <schuchmann@schleissheimer.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>