jtag: usb_blaster: Add missing 'default' to switch statement

If a new JTAG command is added, then GCC will complain that
enumeration value not handled in switch. This is the only driver not
to have a default case, so add it.

Change-Id: Icb838087bb7525d057a911bd256300e256da1668
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://openocd.zylin.com/5333
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/jtag/drivers/usb_blaster/usb_blaster.c

@@ -816,6 +816,11 @@ static int ublast_execute_queue(void)
 		case JTAG_SCAN:
 			ret = ublast_scan(cmd->cmd.scan);
 			break;
+		default:
+			LOG_ERROR("BUG: unknown JTAG command type 0x%X",
+				  cmd->type);
+			ret = ERROR_FAIL;
+			break;
 		}
 	}