tools/kwbimage.c: fix parser error handling

tools/kwbimage.c: fix parser error handling

The two error checks for image_boot_mode_id and image_nand_ecc_mode_id where
wrong and would never fail, fix that!

This was detected by Apple's clang compiler:
---8<---
  HOSTCC  tools/kwbimage.o
tools/kwbimage.c:553:20: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
                if (el->bootfrom < 0) {
                    ~~~~~~~~~~~~ ^ ~
tools/kwbimage.c:571:23: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
                if (el->nandeccmode < 0) {
                    ~~~~~~~~~~~~~~~ ^ ~
2 warnings generated.
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-By: Jeroen Hofstee <jeroen@myspectrum.nl>