update supported hardware list to include NI PXI-6251
update supported hardware list to include NI PXI-6251

Linux-Comedi/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,olsonse/comedilib