Input: psmouse - fix button reporting for basic protocols

The commit ba667650 ("Input: psmouse - clean up code") was pretty
brain-dead and broke extra buttons reporting for variety of PS/2 mice:
Genius, Thinkmouse and Intellimouse Explorer. We need to actually inspect
the data coming from the device when reporting events.

Fixes: ba667650 ("Input: psmouse - clean up code")
Reported-by: Jiri Slaby <jslaby@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>