mt76x2: remove warnings in mt76x2_mac_write_txwi()

Fix following sparse warnings in mt76x2_mac_write_txwi:
- drivers/net/wireless/mediatek/mt76/mt76x2_mac.c:201:26: warning:
  incorrect type in assignment (different base types)
- drivers/net/wireless/mediatek/mt76/mt76x2_mac.c:201:26: expected
  restricted __le32 [usertype] iv
- drivers/net/wireless/mediatek/mt76/mt76x2_mac.c:201:26: got unsigned
  int [unsigned] [usertype] <noident>
- drivers/net/wireless/mediatek/mt76/mt76x2_mac.c:202:27: warning:
  incorrect type in assignment (different base types)
- drivers/net/wireless/mediatek/mt76/mt76x2_mac.c:202:27: expected
  restricted __le32 [usertype] eiv
- drivers/net/wireless/mediatek/mt76/mt76x2_mac.c:202:27: got unsigned
  int [unsigned] [usertype] <noident>

Fixes: 23405236 ("mt76: fix transmission of encrypted management frames")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>