malloc_simple: Remove usage of unsupported %zx format string

Replace %zx by %lx and cast size_t to ulong.

U-Boot currently prints garbage debug output:
size=x, ptr=18, limit=18: 4002a000

With this change it prints correct debug data:
size=18, ptr=18, limit=2000: 4002a000

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>