V4L/DVB (6676): Improve s-code support

s-code tables are related to IF frequency used for video demodulation.

The s-codes for analog are automatically loaded, according with video standard.
However, for digital, they will depend on the IF of the demoduler chip. IF of
the demoduler.

Before this patch, only a few IF's where possible to use. This patch allows
selecting any IF defined at firmware file.

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