Add a function to calculate the manufacturer ID from the 3 byte ASCII
code. The formula can be found at http://www.m-bus.com/files/w4b21021.pdf,
chapter 3.3 at page 6.

This obsoletes the static defines, so remove them.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
mbus_manufacturer_id: add error checking code

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Merge pull request #65 from mhei/manufacturer-id

Add a function to calculate the manufacturer ID from the 3 byte ASCII