Input: atmel_mxt_ts - add support for dynamic message size

The T5 object may have various sizes depending on the objects used on the
particular maXTouch chip and firmware version, therefore it can't be
hardcoded in the driver. Allocate a buffer on probe instead.

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Acked-by: Benson Leung <bleung@chromium.org>
Acked-by: Yufeng Shen <miletus@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>