Input: ams_delta_serio: use private structure

Introduce a driver private structure and allocate it on device probe.
For now, use it instead of a static variable for storing a pointer to
serio structure.  Subsequent patches will populate it with more members
as needed.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>