wifi: mt76: mt7996: introduce mt7996_band_valid()

Replace dbdc_support and tbtc_support with mt7996_band_valid() to
support mt7996 variants with band0/band2 dual-band support.
This changes reduces ambiguity when checking supported bands on different
variants or new chipsets, as well as during the init configurations on
DMA TX/RX queues or irq masks.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>