Input: ads7846 - switch to devm initialization

This simplies the code a lot and fixes some potential resource leaks in
the error return paths.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Link: https://lore.kernel.org/r/20210907200726.2034962-4-daniel@zonque.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>