Input: make gamepad API keycodes more clear

Shuffle the defines around so that it is clear that BTN_A, BTN_B, etc are
legacy definitions and not an accidental typos that need their own key codes.

Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>