sys: net: crosslayer: fixed inet csum for 16bit platforms

[malosek]

Hello,

proposing inet_csum fix for 16bit platforms.
wo explicit type conversion *buff << 8 is treated as signed (checked with gcc-msp430 and TI CGT)

wbr
malo

[kYc0o]

While trying to communicate using a radio attached to an Arduino MEGA 2560 I faced the same problem.
ICMPv6 messages need to be checked using this function so for 8bit processors the explicit cast was also needed.

Code looks good and it works so ACK.

[kYc0o]

Murdock is green -> GO!