jtag: xds110: Add libusb_open() result to error log

When not having access to the usb devices, but using the xds110, the
issue is that one first has to spend time to find the problem. The
additional error message helps others at finding this issue faster.
Solution:
"xds110.c:378 usb_connect(): libusb_open(): LIBUSB_ERROR_ACCESS"

Change-Id: Ic59e29d4dd06868920c2b912e4c866d4f176a563
Signed-off-by: Lucien Dufour <lucien.buchmann@dufour.aero>
Reviewed-on: https://review.openocd.org/c/openocd/+/9265
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/jtag/drivers/xds110.c

@@ -373,6 +373,9 @@ static bool usb_connect(void)
 					/* If we fall though to here, we don't want this device */
 					libusb_close(dev);
 					dev = NULL;
+				} else {
+					const char *err_msg = libusb_error_name(result);
+					LOG_ERROR("libusb_open(): %s", err_msg);
 				}
 			}
 		}