mt76: mt7915: fix endianness warning in mt7915_mac_add_txs_skb

Fix the following sparse warning in mt7915_mac_add_txs_skb routine:

drivers/net/wireless/mediatek/mt76/mt7915/mac.c:1235:29:
	warning: cast to restricted __le32
drivers/net/wireless/mediatek/mt76/mt7915/mac.c:1235:23:
	warning: restricted __le32 degrades to integer

Fixes: 3de4cb17 ("mt76: mt7915: add support for tx status reporting")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>