ath9k_htc: Silence undersized packet warnings

Some devices like TP-Link TL-WN722N produces this kind of messages
frequently.

kernel: ath: phy0: Short RX data len, dropping (dlen: 4)

This warning is useful for developers to recognize that the device
(Wi-Fi dongle or USB hub etc) is noisy but not for general users. So
this patch make this warning to debug message.
Reported-By: Denis <pro.denis@protonmail.com>
Ref: https://bugzilla.kernel.org/show_bug.cgi?id=207539
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/20200504214443.4485-1-masashi.honma@gmail.com

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

@@ -999,9 +999,9 @@ static bool ath9k_rx_prepare(struct ath9k_htc_priv *priv,
 	 * which are not PHY_ERROR (short radar pulses have a length of 3)
 	 */
 	if (unlikely(!rs_datalen || (rs_datalen < 10 && !is_phyerr))) {
-		ath_warn(common,
-			 "Short RX data len, dropping (dlen: %d)\n",
-			 rs_datalen);
+		ath_dbg(common, ANY,
+			"Short RX data len, dropping (dlen: %d)\n",
+			rs_datalen);
 		goto rx_next;
 	}