mtd: rawnand: gpmi: read buf in nand_read_page_op

The driver calls nand_read_page_op without a buffer passed and then
calls chip->legacy.read_buf to read the buffer afterwards which is
the same as passing the buffer nand_read_page_op in the first place.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>