Write kernel log output directly into the xHCI debug port.

One use for this feature is kernel debugging, for example when your
machine crashes very early before the regular console code is
initialized. Other uses include simpler, lockless logging instead of
a full-blown printk console driver + klogd.

For normal production environments this is normally not recommended,
because it doesn't feed events into klogd/syslogd and doesn't try to
print anything on the screen.

You should normally say N here, unless you want to debug early
crashes or need a very simple printk logging facility.