mtd: rawnand: intel: Read the chip-select line from the correct OF node

The chip select has to be read from the flash node which is a child node
of the NAND controller.

Fixes: 0b1039f0 ("mtd: rawnand: Add NAND controller support on Intel LGM SoC")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220702231227.1579176-4-martin.blumenstingl@googlemail.com