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