i2c-cpm: Pass dev ptr to dma_*_coherent rather than NULL

Recent DMA changes result in a BUG() when NULL is passed to
dma_alloc_coherent in place of a device.

Signed-off-by: Mark Ware <mware@elphinstone.net>
[ben-linux@fluff.org: fix patch moves]
Signed-off-by: Ben Dooks <ben-linux@fluff.org>