Input: atmel_mxt_ts - decode T6 status messages

By storing the previous T6 status byte multiple debug output of the same
status can be suppressed (for example CFGERR).

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>