HID: wacom: use devm_kasprintf for allocating the name of the remote

The sysfs group was indeed removed by kobject_put(wacom->remote_dir) in
wacom_remove(), but the name of the group was never freed.

Also remove the misplaced kobject_put(wacom->remote_dir) in the error
path of wacom_remote_create_attr_group().

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>