wifi: mt76: mt7996: fix ldpc setting

wifi: mt76: mt7996: fix ldpc setting

The non-AP interfaces would not use conf->vht_ldpc so they never set
STA_CAP_VHT_LDPC even if peer-station support LDPC.
Check conf->vht_ldpc only for AP interface.

Without this patch, station only uses BCC to transmit packet in VHT mode.

Fixes: dda423dd65c3 ("wifi: mt76: mt7996: remove mt7996_mcu_beacon_check_caps()")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20250114101026.3587702-7-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>