decklink: support all valid numbers of audio channels

As it is already written in the documentation, BMD DeckLink cards
are capable of capturing 2, 8 or 16 audio channels (for SDI Inputs).
Currently the value is hardcoded to 2. Introduces new option.

Reviewed-by: Deti Fliegl <deti@fliegl.de>
Signed-off-by: Matthias Hunstock <atze@fem.tu-ilmenau.de>
Signed-off-by: Marton Balint <cus@passwd.hu>
