arm: zimage: Use correct symbol to hide messages in SPL

When zImage support was added to SPL, the messages were hidden to reduce
code size. However, the wrong config symbol was used. Since this file is
only built when CONFIG_SPL_FRAMEWORK=y, the messages were always hidden.

Use the correct symbol so the messages are printed in U-Boot proper.
Also use IS_ENABLED to drop the #ifdef.

Fixes: 431889d6 ("spl: zImage support in Falcon mode")
Signed-off-by: Samuel Holland <samuel@sholland.org>