mt76x2: init: disable all pending tasklets during device removal

There is a possible race in mt76x2_stop_hardware() since pre_tbtt and
dfs tasklets could run during driver cleanup. Fix it disabling all
pending tasklets during device removal

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>