nvdimm: fix inline function return type warning

nvdimm: fix inline function return type warning

Orabug: 22913653

Fix multiple build warnings when CONFIG_BTT is not enabled:

In file included from ../drivers/nvdimm/bus.c:29:0:
../drivers/nvdimm/nd.h:169:15: warning: return type defaults to 'int' [-Wreturn-type]
 static inline nd_btt_probe(struct nd_namespace_common *ndns, void *drvdata)
               ^

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: linux-nvdimm@lists.01.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
(cherry picked from commit f6ef5a2a50816b58e3126206de13d0b9fdf89df5)
Signed-off-by: Dan Duval <dan.duval@oracle.com>