Bluetooth: Mesh: Fix compiler warning

When building with an LLVM toolchain get the following warning:

bluetooth/mesh/pb_adv.c:774:4: error: expression which evaluates to zero
treated as a null pointer constant of type
'void *' [-Werror,-Wnon-literal-null-conversion]
                        PROV_BEARER_LINK_STATUS_SUCCESS);
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix by adding a cast to (void *).

Signed-off-by: Kumar Gala <kumar.gala@intel.com>