ARM: dma-mapping: avoid calling dma_cache_maint_page() on dev=>cpu

Avoid calling dma_cache_maint_page() when unmapping a DMA_TO_DEVICE
buffer.  The L1 cache ops never do anything in this circumstance, nor
do they ever need to - all that matters for this case is that the data
written is visible to the device before DMA starts.  What happens during
the transfer (provided the buffer is not written to) is of no real
consequence.

We already do this optimisation for the L2 cache.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>