mtd: nand: sunxi: fix return value check in sunxi_nfc_dma_op_prepare()

In case of error, the function dmaengine_prep_slave_sg() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Fixes: 614049a8 ("mtd: nand: sunxi: add support for DMA assisted operations")
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/nand/sunxi_nand.c

@@ -390,8 +390,8 @@ static int sunxi_nfc_dma_op_prepare(struct mtd_info *mtd, const void *buf,
 		return -ENOMEM;
 
 	dmad = dmaengine_prep_slave_sg(nfc->dmac, sg, 1, tdir, DMA_CTRL_ACK);
-	if (IS_ERR(dmad)) {
-		ret = PTR_ERR(dmad);
+	if (!dmad) {
+		ret = -EINVAL;
 		goto err_unmap_buf;
 	}