V4L/DVB (12613): cx25840: fix determining the firmware name

Depending on the model there are three different firmwares to choose from.
Unfortunately if a cx23885 is loaded first, then the global firmware name
is overwritten with that firmware and if ivtv is loaded next, then it
tries to load the wrong firmware. In addition, the original approach would
also overwrite any firmware that the user specified explicitly.

Cc: Jarod Wilson <jarod@wilsonet.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>