[PARISC] fix return type of __atomic64_add_return

The return type of __atomic64_add_return of should be s64 or long, not
int.  This fixes the atomic64 test failure that I previously reported.

Signed-off-by: John David Anglin <dave.anglin@nrc-cnrc.gc.ca>
Cc: stable@kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>