V4L/DVB: get_dvb_firmware: Add function to retrieve nGene firmwares

Commiter: Oliver Endriss <o.endriss@gmx.de>

Use 'get_dvb_firmware ngene' to download 'ngene_15.fw' and 'ngene_17.fw'.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>