[NET]: M32R checksum annotations and cleanups.

* sanitize prototypes, annotate
* ntohs -> shift in checksum calculations in l-e case
* kill shift-by-16 in checksum calculations

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>