powerpc: Don't negate error in syscall_set_return_value()

Currently the only caller of syscall_set_return_value() is seccomp
filter, which is not enabled on powerpc.

This means we have not noticed that our implementation of
syscall_set_return_value() negates error, even though the value passed
in is already negative.

So remove the negation in syscall_set_return_value(), and expect the
caller to do it like all other implementations do.

Also add a comment about the ccr handling.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>