soc/intel/xeon_sp/spr: Refine return value checks

mp_init_with_smm returns cb_err type, where 0 means success and
negative values represent error (see cb_err.h).

However, failure checks in form of "ret < 0" is not
straightforward. Use "ret != CB_SUCCESS" instead.

Change-Id: I7e57f2da0361f3109051e9a35b1cce81d559b261
Signed-off-by: Shuo Liu <shuo.liu@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82210
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>

src/soc/intel/xeon_sp/spr/cpu.c

@@ -271,6 +271,6 @@ void mp_init_cpus(struct bus *bus)
 	intel_microcode_load_unlocked(microcode_patch);
 
 	enum cb_err ret = mp_init_with_smm(bus, &mp_ops);
-	if (ret < 0)
+	if (ret != CB_SUCCESS)
 		printk(BIOS_ERR, "MP initialization failure %d.\n", ret);
 }