mtd: rawnand: diskonchip: Make sure doc2001plus_readbuf() works for single byte reads

Single byte accesses normally go through read_byte() but we are about
to use this function in the exec_op() implementation and thus needs to
prepare for single byte reads.
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/20200501143917.1388957-2-boris.brezillon@collabora.com

drivers/mtd/nand/raw/diskonchip.c

@@ -550,9 +550,12 @@ static void doc2001plus_readbuf(struct nand_chip *this, u_char *buf, int len)
 	}
 
 	/* Terminate read pipeline */
-	buf[len - 2] = ReadDOC(docptr, Mplus_LastDataRead);
-	if (debug && i < 16)
-		printk("%02x ", buf[len - 2]);
+	if (len >= 2) {
+		buf[len - 2] = ReadDOC(docptr, Mplus_LastDataRead);
+		if (debug && i < 16)
+			printk("%02x ", buf[len - 2]);
+	}
+
 	buf[len - 1] = ReadDOC(docptr, Mplus_LastDataRead);
 	if (debug && i < 16)
 		printk("%02x ", buf[len - 1]);