meth: pass struct device to DMA API functions

The DMA API generally relies on a struct device to work properly, and
only barely works without one for legacy reasons.  Pass the easily
available struct device from the platform_device to remedy this.

Also use GFP_KERNEL instead of GFP_ATOMIC as the gfp_t for the memory
allocation, as we aren't in interrupt context or under a lock.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>