mt76: mt7603: enable/disable pre_tbtt_tasklet in mt7603_set_channel

Disable pre_tbtt_tasklet tasklet before setting the operating channel.
Enable/disable beacon_timer in mt7603_set_channel

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>