mt76: mt76x02: set MT_TXOP_HLDR_TX40M_BLK_EN for mt76x2

It needs to be always enabled for 76x2 and conditionally enabled/disabled
for 76x0. Since mt76x2_mac_stop clears this bit, ensure that it is enabled
in mt76x02_edcca_init

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