platform/x86: thinklmi: improved DMI handling

Fix issues reported by kernel test robot.
 - Require DMI for think-lmi.
 - Check return from getting serial string

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506062319.F0IpDxF6-lkp@intel.com/


Signed-off-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250610192830.1731454-1-mpearson-lenovo@squebb.ca


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>