mt76: mt76x02: fix endianness warnings in mt76x02_mac.c

Fix the following sparse warning in mt76x02_mac_write_txwi and
mt76x02_mac_tx_rate_val routines:
drivers/net/wireless/mediatek/mt76/mt76x02_mac.c:237:19:
	warning: restricted __le16 degrades to intege
	warning: cast from restricted __le16
drivers/net/wireless/mediatek/mt76/mt76x02_mac.c:383:28:
	warning: incorrect type in assignment (different base types)
	expected restricted __le16 [usertype] rate
	got unsigned long

Fixes: db9f11d3

 ("mt76: store wcid tx rate info in one u32 reduce locking")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>