NTB: ntb_test: Update ntb_tool DB tests

DB interface of ntb_tool driver hasn't been changed much, but
db_valid_mask DebugFS file has still been added. In this case
it's much better to test all valid DB bits instead of using
the predefined mask, which may be incorrect in general.

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