mtd: rawnand: diskonchip: Get rid of doc2000_readbuf_dword()

The logic can easily be merged in doc2000_readbuf().

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200501143917.1388957-3-boris.brezillon@collabora.com