V4L/DVB (6671): Avoids checking digital/analog at check_firmware

Since check_firmware is called via analog or digital set freq routines, move
type selection to those routines. This avoids having several if's at the code,
and simplifies the source code.

A sideback effect is that implementing radio and other dvb types will become
simpler.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>