soc: qcom: llcc-slice: Add error checks for API functions

llcc_slice_getd can return a ERR_PTR code on failure. Add a IS_ERR_OR_NULL
check to subsequent API calls that use struct llcc_slice_desc to guard
against faults and to let the leaf drivers get away with safely using a
ERR_PTR() encoded "pointer" in the aftermath of a llcc_slice_getd error.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>