km/ivm: fix string len check to support 7 char board names

The fanless boards now have a 7-digit (XXXXX-F) board name. This
triggers a border condition when reading this string in the IVM although
this string is smaller than the currenly read string size, but only by 1
character.

This patch corrects this by changing the size check condition for string
length. It is the same change that was done in the platform for this
same bug.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Stefan Bigler <stefan.bigler@keymile.com>