Input: ad7879 - use more devm interfaces

gpiochip_add now has a managed version, and we can remove sysfs attribute
group via devm_add_action_or_reset (at least until we have devm version of
sysfs_create_group). This allows us to get rid of ad7879_remove().

Reviewed-by: Michael Hennerich <michael.hennerich@analog.com>
Tested-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>