jtag/drivers/stlink_usb: fix SWO prescaler

The config_trace function has an out-parameter for generating the
prescaler for the TPIU. The STLink implementation wasn't always writing
it, causing the tpiu command to load uninitialized stack memory (minus
one) into the TPIU's prescaler register when 'external' was requested.

This change ensures that the out-parameter (and the other one,
trace_freq, which hadn't caused any buggy behavior for me) are written
every time.

Signed-off-by: Cliff L. Biffle <cliff@oxide.computer>
Change-Id: I222975869b1aa49cc6b1963c79d5ea0f46522b8c
Reviewed-on: http://openocd.zylin.com/5656
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/jtag/drivers/stlink_usb.c

@@ -2996,12 +2996,8 @@ int stlink_config_trace(void *handle, bool enabled,
 		return ERROR_FAIL;
 	}
 
-	if (!enabled) {
-		stlink_usb_trace_disable(h);
-		return ERROR_OK;
-	}
-
-	if (*trace_freq > STLINK_TRACE_MAX_HZ) {
+	/* Only concern ourselves with the frequency if the STlink is processing it. */
+	if (enabled && *trace_freq > STLINK_TRACE_MAX_HZ) {
 		LOG_ERROR("ST-LINK doesn't support SWO frequency higher than %u",
 			  STLINK_TRACE_MAX_HZ);
 		return ERROR_FAIL;
@@ -3024,6 +3020,10 @@ int stlink_config_trace(void *handle, bool enabled,
 	}
 
 	*prescaler = presc;
+
+	if (!enabled)
+		return ERROR_OK;
+
 	h->trace.source_hz = *trace_freq;
 
 	return stlink_usb_trace_enable(h);