wifi: mt76: mt7996: Check phy before init msta_link in mt7996_mac_sta_add_links()

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7996: Check phy before init msta_link in mt7996_mac_sta_add_links() In order to avoid a possible NULL pointer dereference in mt7996_mac_sta_init_link routine, move the phy pointer check before running mt7996_mac_sta_init_link() in mt7996_mac_sta_add_links routine.