Input: ALPS - fix forward/back buttons on Ahtec laptop.

I have an Ahtec laptop with a ALPS GlidePoint device, with 4 buttons.
With Linux hernel 2.6.12rc4 and rc5 I'm unable to use the vertical
scroll buttons (BACK and FORWARD).

BACK gets detected as BTN_MIDDLE and FORWARD is undetected.

I've modified the drivers/input/mouse/alps.c from 2.6.12rc5 and now it
works fine!

Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>