mt76: mt76x02: fix coverage_class type

Fix coverage_class type in mt76x02_dev data structure since
coverage_class can be negative to enable dynack (just supported by
ath9k). Set 0 as minimum value for coverage_class

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>