drm_aux-dev: fix error handling in drm_dp_aux_dev_init()

If class_create() fails, there is no need for class_destroy().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1467237135-13075-1-git-send-email-khoroshilov@ispras.ru