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: 0b1039f0

 ("mtd: rawnand: Add NAND controller support on Intel LGM SoC")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210124073955.728797-1-christophe.jaillet@wanadoo.fr