mt76: mt7615: mt7915: disable txpower sku when testmode enabled

When testmode can be enabled, the start() callback would already be
called, causing that txpower sku feature isn't really disabled after
testmode is enabled. This patch fix the issue.

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