ntb: hw: remove the code that sets the DMA mask

This patch removes the code that sets the DMA mask as it no longer
makes sense to do this.

Fixes: 7f46c8b3 ("NTB: ntb_tool: Add full multi-port NTB API support")
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Tested-by: Alexander Fomichev <fomichev.ru@gmail.com>
Signed-off-by: Sanjay R Mehta <sanju.mehta@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>