SPL: fix printing of image name

The maximum length of the name of the image is
obviously not sizeof(), which is just the
length of a pointer, but IH_NMLEN.

fixes: 62cf11c0
("SPL: Limit image name print length")

Signed-off-by: André Draszik <adraszik@tycoint.com>
Acked-by: Simon Glass <sjg@chromium.org>