wifi: mt76: only enable tx worker after setting the channel

Avoids sending packets too early

Fixes: 0b3be9d1 ("wifi: mt76: add separate tx scheduling queue for off-channel tx")
Link: https://patch.msgid.link/20241230194202.95065-8-nbd@nbd.name


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