wifi: mt76: mt7996: fix incorrect interpretation of EHT MCS caps

The EHT MCS map subfield of 20 MHz-Only is not present in the EHT
capability of AP, so STA does not need to parse the subfield.
Moreover, AP should parse the subfield only if STA is 20 MHz-Only, which
can be confirmed by checking supported channel width in HE capability.

Fixes: 92aa2da9 ("wifi: mt76: mt7996: enable EHT support in firmware")
Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Benjamin Lin <benjamin-jw.lin@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>