powerpc/pseries/hvcserver: Fix strncpy buffer limit in location code

the dest buf len is 80 (HVCS_CLC_LENGTH + 1).
  the src buf len is PAGE_SIZE.
  if src buf string len is more than 80, it will cause issue.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>