Currently the data structure for llc-slice is devm allocated and
stored as a global but never cleared if the probe function fails.
This is a problem because devm managed memory gets freed on probe
failure the API functions could access the pointer after it has been
freed.

Initialize the drv_data pointer to an error and reset it to an error
on probe failure or device destroy and add protection to the API
functions to make sure the memory doesn't get accessed.
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>