CMSIS-DAP: print a debug message when the USB product string cannot be read

As suggested by Stian Skjelstad in a comment in:

http://openocd.zylin.com/#/c/2044/

if the USB product string cannot be read, provide a debug message so
users might get aware of a potential permission problem when looking
at the debug output.

Fix style bug found by Jenkins.

Change-Id: I6acb1c6261fec6f2bee80e4be513a5c5e29eff79
Signed-off-by: Jörg Wunsch <openocd@uriah.heep.sax.de>
Reviewed-on: http://openocd.zylin.com/2048
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/cmsis_dap_usb.c

@@ -173,13 +173,18 @@ static int cmsis_dap_usb_open(void)
 	devs = hid_enumerate(0x0, 0x0);
 	cur_dev = devs;
 	while (NULL != cur_dev) {
-		if ((0 == cmsis_dap_vid[0]) && (NULL != cur_dev->product_string)
-		    && wcsstr(cur_dev->product_string, L"CMSIS-DAP")) {
-			/*
-			if the user hasn't specified VID:PID *and*
-			product string contains "CMSIS-DAP", pick it
-			*/
-			break;
+		if (0 == cmsis_dap_vid[0]) {
+			if (NULL == cur_dev->product_string) {
+				LOG_DEBUG("Cannot read product string of device 0x%x:0x%x",
+					  cur_dev->vendor_id, cur_dev->product_id);
+			} else {
+				if (wcsstr(cur_dev->product_string, L"CMSIS-DAP"))
+					/*
+					if the user hasn't specified VID:PID *and*
+					product string contains "CMSIS-DAP", pick it
+					*/
+					break;
+			}
 		} else {
 			/*
 			otherwise, exhaustively compare against all VID:PID in list