spl: fit: Replace #ifdef blocks with more readable constructs

Use the IS_ENABLED() macro to control code flow, instead of the
caveman approach of sprinkling #ifdefs. Code size is not affected, as
the linker garbage-collects unused functions. However, readability is
improved significantly.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>