log: Add a flag to enable log drivers

At present there is no way to disable a log driver. But the syslog driver
causes (attempted) network traffic in sandbox every time a log message
is printed, which is often.

Add a flag to enable a log driver. Adjust struct log_device to use a short
for next_filter_num so that no more memory is used for devices. Also fix
a missing line in the struct log_driver comment while here.

To maintain compatibility, enable it for all drivers for now.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/log_syslog.c

@@ -107,4 +107,5 @@ out:
 LOG_DRIVER(syslog) = {
 	.name	= "syslog",
 	.emit	= log_syslog_emit,
+	.flags	= LOGDF_ENABLE,
 };