ath9k: Fix regression with Atheros 9271

This fix allows ath9k_htc modules to connect to WLAN once again.

Fixes: 2bbcaaee ("ath9k: Fix general protection fault in ath9k_hif_usb_rx_cb")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=208251


Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Reported-by: Roman Mamedov <rm@romanrm.net>
Tested-by: Viktor Jägersküpper <viktor_jaegerskuepper@freenet.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200711043324.8079-1-shiftee@posteo.net