ath9k_htc: reboot firmware if it was loaded

Currently ath9k_htc will reboot firmware only if interface was
ever started. Which lead to the problem in case where interface
was never started but module need to be reloaded.

This patch will partially fix bug "ath9k_htc: Target is unresponsive"
https://github.com/qca/open-ath9k-htc-firmware/issues/1

Reproduction case:
- plug adapter
- make sure nothing will touch it. Stop Networkmanager or blacklist mac address of this adapter.
- rmmod ath9k_htc; sleep 1; modprobe ath9k_htc
Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -1295,7 +1295,9 @@ static void ath9k_hif_usb_disconnect(struct usb_interface *interface)
 
 	usb_set_intfdata(interface, NULL);
 
-	if (!unplugged && (hif_dev->flags & HIF_USB_START))
+	/* If firmware was loaded we should drop it
+	 * go back to first stage bootloader. */
+	if (!unplugged && (hif_dev->flags & HIF_USB_READY))
 		ath9k_hif_usb_reboot(udev);
 
 	kfree(hif_dev);