mt76: mt7615: add multiple wiphy support to the dfs support code

There are two DFS detectors on the chip. When using 160 MHz channel bandwidth
(not supported in dual-wiphy mode), both are used. Otherwise, one detector is
used per wiphy.
Rework the code to start/stop them separately per phy and to indicate the
radar event on the right phy based on the detector index

Signed-off-by: Felix Fietkau <nbd@nbd.name>