mtd: rawnand: stm32_fmc2: fix a buffer overflow

This patch solves following static checker warning:
drivers/mtd/nand/raw/stm32_fmc2_nand.c:350 stm32_fmc2_nfc_select_chip()
error: buffer overflow 'nfc->data_phys_addr' 2 <= 2

The CS value can only be 0 or 1.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Fixes: 2cd457f3 ("mtd: rawnand: stm32_fmc2: add STM32 FMC2 NAND flash controller driver")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1595325127-32693-1-git-send-email-christophe.kerello@st.com