mtd: nand: fix erroneous read_buf call in nand_write_page_raw_syndrome

read_buf is called in place of write_buf in the
nand_write_page_raw_syndrome function.

Signed-off-by: Boris BREZILLON <b.brezillon.dev@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>