NTB: ntb_test: add parameter for doorbell bitmask

If the test attempts to clear doorbell bits that are invalid for the
hardware, then the test will fail.  Provide a parameter to specify the
doorbell bits to clear.  Set default doorbell bits that work for XEON.

Signed-off-by: Allen Hubbe <Allen.Hubbe@dell.com>
Acked-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>