staging: vchiq_arm: Use g_dma_dev for dma_unmap_sg

Commit "staging: vchiq_arm: Clean up 40-bit DMA support" failed to
change one of the calls to dma_unmap_sg to pass in g_dma_dev (rather
than g_dev). Correct that oversight.

See: https://github.com/raspberrypi/linux/issues/3647



Signed-off-by: Phil Elwell <phil@raspberrypi.com>