mt76: mt7615: fix endianness warning in mt7615_mac_write_txwi

Fix the following sparse warning in mt7615_mac_write_txwi routine:
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:758:17:
	warning: incorrect type in assignment
	expected restricted __le32 [usertype]
	got unsigned long

Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Fixes: d4bf77bd ("mt76: mt7615: introduce mt7663u support to mt7615_write_txwi")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>