mtd: OneNAND: OMAP2/3: unmap correct DMA buffer

Functions omap2_onenand_write_bufferram() and omap3_onenand_write_bufferram()
map the write buffer and store the returned handle in variable dma_src. However,
when DMA unmap is done, variable dma_dst is used instead of the correct dma_src.

This patch fixes them to use the correct DMA buffer.

Signed-off-by: Mika Westerberg <ext-mika.1.westerberg@nokia.com>
Tested-by: Arnaud Ebalard <arno@natisbad.org>
Acked-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>