wifi: mt76: mt7915: guard HE capability lookups

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7915: guard HE capability lookups mt7915_mcu_bss_he_tlv() and mt7915_mcu_sta_bfer_tlv() both run after checking HE support, then dereference the HE PHY capability returned by mt76_connac_get_he_phy_cap(). That helper can return NULL when no capability entry matches the vif type. Fetch the capability before appending the TLV and skip the HE-specific setup when no matching capability is available.