soc/amd/picasso/psp: fix check of MSR_PSP_ADDR contents

If MSR_PSP_ADDR is uninitialized, it's all zeros and not all ones.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Iecd3039f63f9d0cb75fe3cb37aee92ba65bbbb50
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50148
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/soc/amd/picasso/psp.c

@@ -14,7 +14,7 @@ void *soc_get_mbox_address(void)
 	uintptr_t psp_mmio;
 
 	psp_mmio = rdmsr(MSR_PSP_ADDR).lo;
-	if (psp_mmio == 0xffffffff) {
+	if (!psp_mmio) {
 		printk(BIOS_WARNING, "PSP: MSR_PSP_ADDR uninitialized\n");
 		return 0;
 	}