NTB: Correct debugfs to work with more than 1 NTB Device

Debugfs was setup in NTB to only have a single debugfs directory.  This
resulted in the leaking of debugfs directories and files when multiple
NTB devices were present, due to each device stomping on the variables
containing the previous device's values (thus preventing them from being
freed on cleanup).  Correct this by creating a secondary directory of
the PCI BDF for each device present, and nesting the previously existing
information in those directories.

Signed-off-by: Jon Mason <jon.mason@intel.com>