mt76: mt7921: fix endianness issues in mt7921_mcu_set_tx()

Fix the following sparse warning in mt7921_mcu_set_tx routine:

drivers/net/wireless/mediatek/mt76/mt7921/mcu.c:792:25: warning: incorrect type in assignment (different base types)
drivers/net/wireless/mediatek/mt76/mt7921/mcu.c:792:25:    expected restricted __le16 [usertype] aifs
drivers/net/wireless/mediatek/mt76/mt7921/mcu.c:792:25:    got unsigned char [usertype] aifs
drivers/net/wireless/mediatek/mt76/mt7921/mcu.c:798:35: warning: incorrect type in assignment (different base types)
drivers/net/wireless/mediatek/mt76/mt7921/mcu.c:798:35:    expected restricted __le16 [usertype] cw_min

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>