mt76: mt7615: fix sparse warnings: warning: restricted __le16 degrades to integer

Fix the following sparse warning in __mt7615_mcu_msg_send:
drivers/net/wireless/mediatek/mt76/mt7615/mcu.c:78:15: sparse: warning:
restricted __le16 degrades to integer
drivers/net/wireless/mediatek/mt76/mt7615/mcu.c:78:15: sparse: warning:
cast from restricted __le16

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