All error codes should be const. Most are, but there are
lots of places where we forgot to add <constant> at the DocBook.

Fix those via this small script:
	for i in $(git grep -lE "\s+E[A-Z]+\b" Documentation/linux_tv/); do perl -ne 's,([^\`])\b(E[A-Z]+)\b,\1``\2``,g; print $_' <$i >a && mv a $i; done

As there are false positives, we needed to merge only the changes
that make sense, skipping the c blocks and skipping things like
EDID, EN, ETS that were also converted by the above code.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>