slimbus: fix a potential NULL pointer dereference in of_qcom_slim_ngd_register

In case platform_device_alloc fails, the fix returns an error
code to avoid the NULL pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>