microblaze: clean up checksum.c

This changes the function prototypes in the checksum code
to have the usual prototypes, typically by turning int
arguments into __wsum.

Also change csum_partial_copy_from_user() to operate
on the right address space and export ip_fast_csum,
which is used in modular networking code.

The new version is now sparse-clean including endianess
checks.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <monstr@monstr.eu>