The return value for decompress_kernel() is no longer used.  Furthermore,
this was obtained and stored in a variable called output_ptr which is
a complete misnomer for what is actually the size of the decompressed
kernel image.  Let's get rid of it.
Signed-off-by: NNicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: NShawn Guo <shawn.guo@linaro.org>
Tested-by: NTony Lindgren <tony@atomide.com>