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_USER as the gfp_t for the memory
allocation, as we should treat this allocation as a normal kernel one.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>