mtd: rawnand: intel: Fix an error handling path in 'ebu_dma_start()'

If 'dmaengine_prep_slave_single()' fails, we must undo a previous
'dma_map_single()' call, as already done in all the other error handling
paths of this function.

Fixes: 0b1039f016e8 ("mtd: rawnand: Add NAND controller support on Intel LGM SoC")
Signed-off-by: Christophe JAILLET <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/nand/raw/intel-nand-controller.c

···
 	}
 
 	tx = dmaengine_prep_slave_single(chan, buf_dma, len, dir, flags);
-	if (!tx)
-		return -ENXIO;
+	if (!tx) {
+		ret = -ENXIO;
+		goto err_unmap;
+	}
 
 	tx->callback = callback;
 	tx->callback_param = ebu_host;