nand: stm32_fmc2_nand: correct xor_ecc.val assigned value

The variable is wrongly set to 0L, whereas it is an unsigned int, it should
then be 0U.

Change-Id: I0b164c0ea598ec8a503f1693da2f3789f59da238
Signed-off-by: Yann Gautier <yann.gautier@st.com>