Input: psmouse - rework setting of BTN_MIDDLE capability

Do not start protocol detection assuming that middle mouse is present,
instead let individual protocols explicitly set this capability.
This fixes issue with Synaptics touchpads pretending that they have
middle button when hardware clearly reports otherwise.

Reported-and-tested-by: Andrey Borzenkov <arvidjaar@mail.ru>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>