mtd: omap2: fix resource leak in prefetch-busy path

If prefetch engine is busy, current code "forgets" to call
dma_unmap_single(), which results in a deadlock later, so add it.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>