wifi: mt76: mt7915: rework radar HWRDD idx

wifi: mt76: mt7915: 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.
Therefore, add mt7915_get_rdd_idx as a helper function to get the
HWRDD idx for each variants.

Additionally, 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(=bandidx)| Background rdd idx
-------------------------------|---------------------|-------------------
MT7915A                        | 0                   | 2
MT7915D                        | 1                   | 2
MT7916 2G + 5G (2T2R+1R)       | 1                   | 2
MT7916 2G + 5G (3T3R)          | 1                   | N/A
MT7981 2G + 5G                 | 1                   | N/A
MT7986 2G + 5G (one adie DBDC) | 1                   | N/A
MT7986 5G (one adie)           | 1 (bandidx=MT_BAND1)| N/A
MT7986 2G + 5G (dual adie)     | 1                   | N/A

Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20250320015918.3948643-1-StanleyYP.Wang@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>