wm831x_backup: Fix wrong kfree call for devdata->backup.name

devdata->backup.name points to devdata->name, the memory for devdata->name
is part of struct wm831x_backup. Thus remove kfree call for
devdata->backup.name.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>