Input: synaptics - update OLPC XO exclusion

We have determined that the jumpiness previously seen when using
the synaptics kernel mouse driver on OLPC XO was due to not using
the synaptics X11 userspace driver - the xf86-input-evdev driver was
interpreting 'finger near pad' signals as movements. Newer versions
of xf86-input-evdev fix this issue.

Additionally, the synaptics kernel driver is now usable on this
platform, but only when run in relative mode.

Update the comment and refine the check to allow the synaptics driver
to run on OLPC XO in relative mode.

We will continue investigating the EC issue as time becomes available.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>