wifi: mt76: mt7915: fix power-limits while chan_switch

wifi: mt76: mt7915: fix power-limits while chan_switch

If user changes the channel without completely disabling the interface the
txpower_sku values reported track the old channel the device was operating on.
If user bounces the interface the correct power tables are applied.

mt7915_sku_group_len array gets updated before the channel switch happens so it
uses data from the old channel.

Fixes: ecb187a74e18 ("mt76: mt7915: rework the flow of txpower setting")
Fixes: f1d962369d56 ("mt76: mt7915: implement HE per-rate tx power support")
Reported-By: Chad Monroe <chad.monroe@smartrg.com>
Tested-by: Chad Monroe <chad.monroe@smartrg.com>
Signed-off-by: Allen Ye <allen.ye@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>