mt76: fix possible OOB issue in mt76_calculate_default_rate

Cap max offset value to ARRAY_SIZE(mt76_rates) - 1 in
mt76_calculate_default_rate routine in order to avoid possible Out Of
Bound accesses.

Fixes: 33920b2b ("mt76: add support for setting mcast rate")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>