In the Dell XPS 13 9333, it appears that sometimes the bus get confused
and corrupts the incoming data. It fills the input report with the
sentinel value "ff". Synaptics told us that such behavior does not comes
from the touchpad itself, so we filter out such reports here.

Unfortunately, we can not simply discard the incoming data because they
may contain useful information. Most of the time, the misbehavior is
quite near the end of the report, so we can still use the valid part of
it.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1123584Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>