NTB: Set dma mask and dma coherent mask to NTB devices

The dma_mask and dma_coherent_mask fields of the NTB struct device
weren't initialized in hardware drivers. In fact it should be done
instead of PCIe interface usage, since NTB clients are supposed to
use NTB API and left unaware of real hardware implementation.
In addition to that ntb_device_register() method shouldn't clear
the passed ntb_dev structure, since it dma_mask is initialized
by hardware drivers.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>