drivers/misc/sgi-xp: replace partid_t with a short

In preparation for supporting greater than 64 partitions replace partid_t by
short in drivers/misc/sgi-xp.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Acked-by: Robin Holt <holt@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>