sim: Use `usize` instead of `u8` for alignment

Some devices have a write alignment larger than 128.  Allow this within
the simulator, by using `usize` instead of `u8` for the write alignment.
The value is still returned to the C code as an 8-bit integer, but this
help start debugging issues found on these devices.

Signed-off-by: David Brown <david.brown@linaro.org>