wifi: mt76: mt7915: guard HE capability lookups

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.

Fixes: e6d557a78b60 ("mt76: mt7915: rely on mt76_connac_get_phy utilities")
Signed-off-by: Ruoyu Wang <ruoyuw560@gmail.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260620155332.81120-1-ruoyuw560@gmail.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>