iwlwifi: mvm: add debugfs entry to read the current temperature

When read, this new debugfs entry requests the current temperature
from the firmware and outputs it.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>