[regression fix] really dumb fuckup in sparc64 __csum_partial_copy() changes

~0U is -1, not 1

Reported-by: Anatoly Pugachev <matorola@gmail.com>
Tested-by: Anatoly Pugachev <matorola@gmail.com>
Fixes: fdf8bee9 "sparc64: propagate the calling convention changes down to __csum_partial_copy_...()"
X-brown-paperbag: yes
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>