flash/nor/kinetis: fix clang scan-build error format-truncation

flash/nor/kinetis: fix clang scan-build error format-truncation

kinetis.c:994:61: error: '%u' directive output may be truncated
writing between 1 and 10 bytes into a region of size 4
[-Werror=format-truncation=]

tested with scan-build-14

Signed-off-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Change-Id: I72d141a3f8e19ca3596beee2be8434fc8492946f
Reviewed-on: https://review.openocd.org/c/openocd/+/7140
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins