xhci: fix garbage USBSTS being logged in some cases

xhci_decode_usbsts() is expected to return a zero-terminated string by
its only caller, xhci_stop_endpoint_command_watchdog(), which directly
logs the return value:

  xhci_warn(xhci, "USBSTS:%s\n", xhci_decode_usbsts(str, usbsts));

However, if no recognized bits are set in usbsts, the function will
return without having called any sprintf() and therefore return an
untouched non-zero-terminated caller-provided buffer, causing garbage
to be output to log.

Fix that by always including the raw value in the output.

Note that before commit 4843b4b5ec ("xhci: fix even more unsafe memory
usage in xhci tracing") the result effect in the failure case was different
as a static buffer was used here, but the code still worked incorrectly.

Fixes: 9c1aa36efd ("xhci: Show host status when watchdog triggers and host is assumed dead.")
Cc: stable@vger.kernel.org
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20220303110903.1662404-3-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.h

@@ -2625,8 +2625,11 @@ static inline const char *xhci_decode_usbsts(char *str, u32 usbsts)
 {
 	int ret = 0;
 
+	ret = sprintf(str, " 0x%08x", usbsts);
+
 	if (usbsts == ~(u32)0)
-		return " 0xffffffff";
+		return str;
+
 	if (usbsts & STS_HALT)
 		ret += sprintf(str + ret, " HCHalted");
 	if (usbsts & STS_FATAL)