Input: use svg files instead of xfig in force feedback documentation

Now that the images got converted to svg, add them at the
ff.rst file.

NOTE: After merging upstream, the "figure" tag should be
replaced by the new "kernel-figure" tag, in order for the
SVG images to be included at the PDF files.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>