mt76x2: apply coverage class on slot time too

mt76x2: apply coverage class on slot time too

According to 802.11-2007 17.3.8.6 (slot time), the slot time should
be increased by 3 us * coverage class. Taking into account coverage
class in slot time configuration allows to increase by an order of
magnitude the throughput on a 4Km link in a noisy environment

Tested-by: Luca Bisti <[email protected]>
Tested-by: Gaetano Catalli <[email protected]>
Signed-off-by: Lorenzo Bianconi <[email protected]>
Acked-by: Felix Fietkau <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>