image: Avoid switch default in image_decomp()

image: Avoid switch default in image_decomp()

At present this function is full of preprocessor macros. Adjust it to
check for an unsupported algorithm after the switch(). This will allow
us to drop the macros.

Fix up the return-value path and an extra blank line while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>