tools: Correct error handling in fit_image_process_hash()

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/[email protected]/msg217417.html

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

Signed-off-by: Simon Glass <[email protected]>
Reported-by: Tom Van Deun <[email protected]>
Reviewed-by: Teddy Reed <[email protected]>