Input: psmouse - wheel mice (imps, exps) always have 3rd button

There are wheel mice that respond to Logitech probes and report
that they have only 2 buttons (such as e-Aser mouse) and this
stops the wheel from being used as a middle button. Change the
driver to always report BTN_MIDDLE capability if a wheel is
present.

Also, never reset BTN_RIGHT capability in logips2pp code - there
are no Logitech mice that have only one button and if some other
mice happen to respond to Logitech's query we could do the wrong
thing.

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