tools: Correct error handling in fit_image_process_hash()

We should not be returning -1 as an error code. This can mask a situation
where we run out of space adding things to the FIT. By returning the correct
error in this case (-ENOSPC) it can be handled by the higher-level code.

This may fix the error reported by Tom Van Deun here:

https://www.mail-archive.com/u-boot@lists.denx.de/msg217417.html



although I am not sure as I cannot actually repeat it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Tom Van Deun <tom.vandeun@wapice.com>
Reviewed-by: Teddy Reed <teddy.reed@gmail.com>