libnvdimm-btt: Delete an unnecessary check before the function call "__nd_device_register"

The __nd_device_register() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>