dmaengine: Fixup dmaengine_prep_slave_single() to be actually useful

dmaengine_prep_slave_single() is a helper function which is supposed to be used
to prepare a transfer of a single contingous buffer. Currently the function
takes a pointer to such a buffer from which it builds a scatterlist and passes
it on to device_prep_slave_sg. The dmaengine framework requires that any
scatterlist that is passed to device_prep_slave_sg is mapped and it may not be
unmapped until the DMA operation has completed. This is not the here and any use
of dmaengine_prep_slave_single() will lead to undefined behaviour (Most likely a
system crash).

This patch changes dmaengine_prep_slave_single() to take a dma_addr_t instead of
a pointer to a buffer and moves the responsibility of mapping and unmapping the
buffer up to the caller.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>