Input: synaptics - adjust threshold for treating position values as negative

Commit c0394506 (Input: synaptics - handle out of bounds values from the
hardware) caused any hardware reported values over 7167 to be treated as
a wrapped-around negative value. It turns out that some firmware uses
the value 8176 to indicate a finger near the edge of the touchpad whose
actual position cannot be determined. This value now gets treated as
negative, which can cause pointer jumps and broken edge scrolling on
these machines.

I only know of one touchpad which reports negative values, and this
hardware never reports any value lower than -8 (i.e. 8184). Moving the
threshold for treating a value as negative up to 8176 should work fine
then for any hardware we currently know about, and since we're dealing
with unspecified behavior it's probably the best we can do. The special
8176 value is also likely to result in sudden jumps in position, so
let's also clamp this to the maximum specified value for the axis.

BugLink: http://bugs.launchpad.net/bugs/1046512
https://bugzilla.kernel.org/show_bug.cgi?id=46371



Cc: stable@vger.kernel.org
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Alan Swanson <swanson@ukfsn.org>
Tested-by: Arteom <arutemus@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>