Input: ad7879 - use managed devm_device_add_group

Commit 57b8ff07 ("driver core: add devm_device_add_group() and
friends") has added the managed version for creating sysfs group files.

Use devm_device_add_group instead of sysfs_create_group and remove the
action that cleans the sysfs file when exiting the driver.

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