soc: qcom: ocmem: don't return NULL in of_get_ocmem

If ocmem probe fails for whatever reason, of_get_ocmem returned NULL.
Without this, users must check for both NULL and IS_ERR on the returned
pointer - which didn't happen in drivers/gpu/drm/msm/adreno/adreno_gpu.c
leading to a NULL pointer dereference.

Reviewed-by: Brian Masney <masneyb@onstation.org>
Fixes: 88c1e940

 ("soc: qcom: add OCMEM driver")
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lore.kernel.org/r/20210130142349.53335-1-luca@z3ntu.xyz
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>