wifi: mt76: mt7996: fix NULL pointer dereference in mt7996_mcu_sta_bfer_he

Fix the NULL pointer dereference in mt7996_mcu_sta_bfer_he
routine adding an sta interface to the mt7996 driver.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://patch.msgid.link/20240813081242.3991814-1-make24@iscas.ac.cn


Signed-off-by: Felix Fietkau <nbd@nbd.name>