spl: use CONFIG_IS_ENABLED(LOAD_FIT_FULL)

spl: use CONFIG_IS_ENABLED(LOAD_FIT_FULL)

It's possible that LOAD_FIT_FULL will have different values for TPL and
SPL, in which case just checking CONFIG_SPL_LOAD_FIT_FULL causes this to
be compiled in to the TPL even though functions and struct members it
depends on are not.

Use CONFIG_IS_ENABLED() to ensure the correct TPL/SPL variant is
checked.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>