Input: wm97xx-core - simplify error path in wm97xx_probe()

Use platform_device_del() instead of platform_device_unregister() in error
handling path.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Acked-by: Mark Brown <broonie@opensoruce.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>