Input: atmel_mxt_ts - split config update a bit

Let's split config update code a bit so it is hopefully a bit easier to
read. Also, the firmware update callback should be the entity releasing
firmware blob, not lower layers.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>