board: stm32mp1: correct CONFIG_IS_ENABLED usage for LED

Use the correct macro to test presence CONFIG_LED:
replace CONFIG_IS_ENABLED(CONFIG_LED) by CONFIG_IS_ENABLED(LED)

Issue see during review unrelated patch
"board: stm32mp1: update management of boot-led"
http://patchwork.ozlabs.org/patch/1264823/



Cc: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>