openni_device: return prime sense serial id
openni_driver: return prime sense serial id
workaround: remove some calls that segfault later

unless this patch is applied, the driver will segfault later
when querying the prime sense serial id.

the fact that this patch is necessary is somewhat fishy and
an indication that the other patches are probably not quite
correct.
Merge pull request #16 from forouher/asus-serial

ASUS xtion: use serial number