mailbox: PCC: Fix return value of pcc_mbox_request_channel()

When CONFIG_PCC is disabled, pcc_mbox_request_channel() needs to
return ERR_PTR(-ENODEV), not a NULL pointer, as the callers of
this function use IS_ERR() to check for error code.

Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>