wifi: mt76: mt7996: rework radar HWRDD idx

wifi: mt76: mt7996: rework radar HWRDD idx

The definition of MT_RX_SEL (for rdd_rx_sel) is mixed with the
definition of HWRDD idx.
For example, MT_RX_SEL2 is for background HWRDD idx, not an
option of rdd_rx_sel.
Additionally, HWRDD idx does not exactly map to band idx for
Connac 3 chips. So, add mt7996_get_rdd_idx as a helper function.

Finally, remove some parts of the code inherited from the legacy chips.
For instance,
1. rdd_state is used for single-band-dual-HWRDD chips (for 80+80),
especially the 76xx series.
2. rdd_rx_sel is also used for single-band-dual-HWRDD chips
rx_sel = 0 => RDD0 for WF0, RDD1 for WF2
rx_sel = 1 => RDD0 for WF1, RDD1 for WF3

Chip Variants              | 5G rdd idx     | Background rdd idx
---------------------------|----------------|-------------------
MT7996 (except 205/255)    | 1              | 2
MT7992                     | 1              | 2
MT7990                     | 1              | 2

Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Reviewed-by: Money Wang <money.wang@mediatek.com>
Link: https://patch.msgid.link/20250320015926.3948672-1-StanleyYP.Wang@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>