s390/virtio_ccw: avoid converting dma addresses / handles

Instead of converting virtual to physical addresses with the virt_to_dma*()
functions, use dma addresses as provided by DMA API and only add offsets to
these addresses. This makes sure that address conversion is only done by
the DMA API.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>