wifi: iwlwifi: mvm: fix warnings from dmi_get_system_info()

wifi: iwlwifi: mvm: fix warnings from dmi_get_system_info()

dmi_get_system_info() will statically return NULL when the
kernel is compiled without CONFIG_DMI, leading to compiler
warnings. Fix that by printing "<unknown>" in that case.

Fixes: c3f40c3e0273 ("iwlwifi: mvm: add US/CA to TAS block list if OEM isn't allowed")
Fixes: 9457077df49e ("wifi: iwlwifi: mvm: Add debugfs to get TAS status")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>