mt76: mt7921: fix endianness warning in mt7921_update_txs

Fix the following sparse warning in mt7921_update_txs routine:
drivers/net/wireless/mediatek/mt76/mt7921/mac.c:752:31:
	warning: cast to restricted __le32
drivers/net/wireless/mediatek/mt76/mt7921/mac.c:752:31:
	warning: restricted __le32 degrades to integer

Fixes: e5bca8c5 ("mt76: mt7921: improve code readability for mt7921_update_txs")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>