mt76: do not enable/disable pre_tbtt_tasklet in scan_start/scan_complete

Do not enable/disable pre_tbtt_tasklet tasklet in
mt76x02_sw_scan/mt76x02_sw_scan_complete since it is already done
setting the operating channel. Do run tbtt_tasklet while the device is
offchannel

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