[media] siano: simplify firmware lookup logic

There are two ways to specify firmware for siano devices: a
per-device ID and a per-device type.
The per-device type logic is currently made by a 11x9 string
table, sparsely filled. It is very hard to read the table at
the source code, as there are too much "none" filling there
("none" there is a way to tell NULL).
Instead of using such problematic table, convert it into an
easy to read table, where the unused values will be defaulted
to NULL.
While here, also simplifies a little bit the logic and print
a message if an user-selected mode doesn't exist.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>