tile: fix a -Wframe-larger-than warning

The warning occurs in setup.c, where it is known that it can't be
a problem, but it's still a good idea to silence the warning.
The onstack array is converted from an s32 to a u8, which still
is plenty of range for the values being managed there.

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