frame: change checksum function

frame: change checksum function

At the end of function, we used to call ntohs() but we also call the
same function before providing the checksum to POKE_UINT16 (which will
in turn call ntohs). We remove those two first extra calls as they
aren't necessary.

Moreover, the SNMP agent uses this function and this makes it
endian-dependant. We don't want that.

The function is quite convulated and isn't a classic checksum
function. I don't remember how I came with this function but since there
is the special case for Cisco devices and I know this special case work
now, I don't want to change to another function which may breaks this
special case.

Add a NEWS entry to let people know, just in case.