csum: fixed for 16bits platforms
csum: fixed for 16bits platforms

Signed-off-by: malo <bf7cdc72adfdcf4153c1572af5b7953bc9e9c4c6@25cmsquare.io>

malosek/RIOT,malosek/RIOT,malosek/RIOT,malosek/RIOT,malosek/RIOT,malosek/RIOT