drivers/misc/sgi-xp: clean up return values

Make XP return values more generic to XP and not so tied to XPC by changing
enum xpc_retval to xp_retval, along with changing return value prefixes from
xpc to xp.  Also, cleanup a comment block that referenced some of these return
values as well as the handling of BTE related return values.

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>