Input: ad7877 - use managed resource allocations

Use managed resource allocations to simplify error handling during
probing failure and module exiting.

With this all the goto labels in the probe function together with
the cleanups in the remove function are unnecessary, therefore
removed.

Signed-off-by: Andi Shyti <andi@etezian.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>