Input: atkbd - release previously reserved keycodes 248 - 254

Keycodes in 248 - 254 range were reserved for special needs (scrolling)
of atkbd driver. Now that the driver has been switched to use unsigned
short keycodes instead of unsigned char we can release this range back
into pull. We keep code 255 (ATKBD_KEY_NULL) reserved since users may
have been using it to silence keys they do not care about since atkbd
silently drops scancodes mapped to this keycode.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>