wifi: mt76: mt7996: fix beacon command during disabling

wifi: mt76: mt7996: fix beacon command during disabling

When stopping AP, mac80211 frees beacon template before it calls
driver's stop_ap operation. In other words, on the path of stopping
AP, ieee80211_beacon_get_template() must returns NULL in
mt7996_mcu_add_beacon(). In such case mt7996 immediately returns
-EINVAL without telling FW to disable the beacon.

This commit refactors mt7996_mcu_add_beacon() so that FW can be
correctly informed when disabling AP interface.

Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Michael-CY Lee <michael-cy.lee@mediatek.com>
Link: https://patch.msgid.link/20250114101026.3587702-8-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>