scripts: kconfig: fix dt_nodelabel_int_prop crash if prop doesn't exist

dt_nodelabel_int_prop did not check if the property existed on the node
before accessing it, which would result in a build-time crash (and thus
build error) if used on a node which lacked the requested property.

Fix by using the common _node_int_prop() helper which handles all edge
cases properly.

Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>