Input: alps - fix touchpad buttons getting stuck when used with trackpoint

When the left touchpad button gets pressed, and then the trackpoint is
moved, and then the button is released, the following happens:

1) touchpad packet is received, touchpad evdev node reports BTN_LEFT 1

2) pointing stick packet is received, the hw will report a BTN_LEFT 1 in
   this packet because when the trackstick is active it communicates the
   combined touchpad + pointing stick buttons in the trackstick packet,
   since alps_report_bare_ps2_packet passes NULL (*) for the dev2 parameter
   to alps_report_buttons the combining is not detected and the
   pointing stick evdev node will also report BTN_LEFT 1

3) on release of the button a pointing stick packet with BTN_LEFT 0 is
   received and the pointing stick evdev node will report BTN_LEFT 0

Note how because of the passing as NULL for dev2 the touchpad evdev node
will never send BTN_LEFT 0 in this scenario leading to a stuck mouse button.

This is a regression in 4.0 introduced by commit 04aae283
("Input: ALPS - do not mix trackstick and external PS/2 mouse data")

This commit fixes this by passing in the touchpad evdev as dev2 parameter
when calling alps_report_buttons for the pointingstick on alps v2 devices,
so that alps_report_buttons correctly detect that we're already reporting
the button as pressed via the touchpad evdev node, and will also send the
release event there.

Cc: stable@vger.kernel.org # 4.0
Reported-by: Hans de Bruin <jmdebruin@xmsnet.nl>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>