wifi: mt76: mt7996: fix endianness warning in mt7996_mcu_sta_he_tlv

Fix the following sparse warnings in mt7996_mcu_sta_he_tlv routine:

warning: incorrect type in assignment (different base types)
   expected unsigned char
   got restricted __le16 [usertype]
warning: incorrect type in assignment (different base types)
   expected unsigned char
   got restricted __le16 [usertype]

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>