Input: synaptics - handle spurious release of trackstick buttons

The Fimware 8.1 has a bug in which the extra buttons are only sent when the
ExtBit is 1.  This should be fixed in a future FW update which should have
a bump of the minor version.

Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>