altos: AES code was storing bytes in an int array

altos: AES code was storing bytes in an int array

The only operations using these values are logic using bytes, not
arithmetic, so shrinking the size of the elements should have no
effect on operation.

This saves space *and* eliminates a type conversion warning from
-Wconversion.

Signed-off-by: Keith Packard <keithp@keithp.com>