mtd: rawnand: fsmc: Stop using chip->read_buf()

chip->read_buf is left unassigned since commit 4da712e7 ("mtd: nand:
fsmc: use ->exec_op()"), leading to a NULL pointer dereference when it's
called from fsmc_read_page_hwecc(). Fix that by using the appropriate
helper to read data out of the NAND.

Fixes: 4da712e7 ("mtd: nand: fsmc: use ->exec_op()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>