mailbox: pcc: Fix an invalid-load caught by the address sanitizer

`pcc_mailbox_probe` doesn't initialize all memory that has been allocated
before the first time that one of it's members `txdone_irq` may be
accessed.

This leads to a an invalid load any time that this member is accessed:
[    2.429769] UBSAN: invalid-load in drivers/mailbox/pcc.c:684:22
[    2.430324] UBSAN: invalid-load in drivers/mailbox/mailbox.c:486:12
[    4.276782] UBSAN: invalid-load in drivers/acpi/cppc_acpi.c:314:45

Link: https://bugzilla.kernel.org/show_bug.cgi?id=215587
Fixes: ce028702dd ("mailbox: pcc: Move bulk of PCCT parsing into pcc_mbox_probe")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/pcc.c

@@ -654,7 +654,7 @@ static int pcc_mbox_probe(struct platform_device *pdev)
 		goto err;
 	}
 
-	pcc_mbox_ctrl = devm_kmalloc(dev, sizeof(*pcc_mbox_ctrl), GFP_KERNEL);
+	pcc_mbox_ctrl = devm_kzalloc(dev, sizeof(*pcc_mbox_ctrl), GFP_KERNEL);
 	if (!pcc_mbox_ctrl) {
 		rc = -ENOMEM;
 		goto err;