V4L/DVB (6659): Convert MTS to bitfield

Xc2028.3028 has two type of firmwares: audio-standard specific ones and
baseband MTS firmwares. MTS firmwares provide stereo decoding for 6 MHz
BTSC/EIAJ and for monoaural audio decoding on 8 MHz firmwares.

It seems that the option to use MTS or a standard-specific audio decoding
depends on the way xc2028/3028 is connected.

Instead of wasting 32 (or 64 bits) to signalize if the driver needs to use MTS
firmware, this patch converts it to a bitfield that can be shared with other
proprieties of xc2028/3028.

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