tile: fix some -Wsign-compare warnings

Normally the build doesn't include these warnings, but at one
point I built with -Wsign-compare, and noticed a few things that
are technically bugs.  This change fixes those things.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

diff --cc arch/tile/include/asm/io.h
Simple merge

diff --cc arch/tile/include/asm/mmzone.h
Simple merge

diff --cc arch/tile/include/asm/spinlock_64.h
Simple merge