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

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: ce028702ddbc ("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>