mt76: add energy detect CCA support to mt76x{0,2}e drivers

Ported from the reference driver. Should fix compliance with ETSI
regulatories on preventing transmission while energy detect values
are above the threshold.
The code has been tested using an ath9k device running tx99 as
noise generator

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>