tools: Fix return code of fit_image_process_sig()

tools: Fix return code of fit_image_process_sig()

When signing images, we repeatedly call fit_add_file_data() with
successively increasing size values to include the keys in the DTB.

Unfortunately, if large keys are used (such as 4096 bit RSA keys), this
process fails sometimes, and mkimage needs to be called repeatedly to
integrate the keys into the DTB.

This is because fit_add_file_data actually returns the wrong error
code, and the loop terminates prematurely, instead of trying again with
a larger size value.

This patch corrects the return value and also removes a error message,
which is misleading, since we actually allow the function to fail. A
(hopefully helpful) comment is also added to explain the lack of error
message.

This is probably related to 1152a05 ("tools: Correct error handling in
fit_image_process_hash()") and the corresponding error reported here:

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

Signed-off-by: Mario Six <mario.six@gdsys.cc>