work around xmlto bug in htmldocs

Trying to generate xhtml causes all functions to show up with a prefix
of "fsfunc" in the output, so just back off to html until someone
fixes the toolchain.

Note that this is not a problem with kernel-doc, it's an issue with
however "xmlto" renders xhtml output.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>