msm: kgsl: Make the stubbed version of cff_dump return -ENODEV

Currently the stubbed version of the cff_dump debugfs file returns
-EINVAL which is a ultra overloaded error message.  Switch it to
-NODEV to communicate more clearly to the GPU that the functionality
isn't available.

Change-Id: Ic0dedbad780497b8803f09e128c5841e9aa14f28
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>