platform/x86/amd/pmc: Avoid logging "(null)" for DMI values

platform/x86/amd/pmc: Avoid logging "(null)" for DMI values

dmi_get_system_info(...) can return NULL. Using that as %s arguments
of dev_info() would log "(null)" (as part of a message like
'... System Vendor: "(null)", Product Name: "(null)" ...'), which may
be confusing for users.

Use Elvis operator to print "(Unknown)" instead.

Fixes: 428b9fd2dce5 ("platform/x86/amd/pmc: Add delay_suspend module parameter")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202606251540.Nr2BtaNu-lkp@intel.com/
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Daniel Gibson <daniel@gibson.sh>
Link: https://patch.msgid.link/20260626220210.1761783-2-daniel@gibson.sh
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>