kwbimage.c: Fix compile warning when building on 64 bit systems (again)

Commit 51003b89 attempted to fix a build problem on 64 bit systems,
but just turned it into a build problem on 32 bit systems (silly me).

Now do the Right Thing (TM) and use a "%zu" printf format.

Also fix spelling error.

Signed-off-by: Wolfgang Denk <wd@denx.de>