ath9k_htc: Use appropriate rs_datalen type

kernel test robot says:
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:987:20: sparse: warning: incorrect type in assignment (different base types)
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:987:20: sparse:    expected restricted __be16 [usertype] rs_datalen
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:987:20: sparse:    got unsigned short [usertype]
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:988:13: sparse: warning: restricted __be16 degrades to integer
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:1001:13: sparse: warning: restricted __be16 degrades to integer

Indeed rs_datalen has host byte order, so modify it's own type.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: cd486e62 ("ath9k_htc: Discard undersized packets")
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200808233258.4596-1-masashi.honma@gmail.com

drivers/net/wireless/ath/ath9k/htc_drv_txrx.c

@@ -974,7 +974,7 @@ static bool ath9k_rx_prepare(struct ath9k_htc_priv *priv,
 	struct ath_htc_rx_status *rxstatus;
 	struct ath_rx_status rx_stats;
 	bool decrypt_error = false;
-	__be16 rs_datalen;
+	u16 rs_datalen;
 	bool is_phyerr;
 
 	if (skb->len < HTC_RX_FRAME_HEADER_SIZE) {