ntb: ntb_pingpong: no need to check the return value of debugfs calls

There is no need to check the return value of debugfs_create_atomic_t as
nothing happens with the error.  Also, the code will never return NULL,
so this check has never caught anything :)

Fix this by removing the check entirely.

Cc: Jon Mason <jdmason@kudzu.us>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Allen Hubbe <allenbh@gmail.com>
Cc: linux-ntb@googlegroups.com
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20191011131919.GA1174815@kroah.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>