iwlwifi: mvm: return -ENODATA when reading the temperature with the FW down

It seems that libsensors treats -EIO as a special non-recoverable
failure when it tries to read the temperature while the firmware is
not running.  To solve that, change the error code to a milder
-ENODATA.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=196941



Fixes: c221daf2 ("iwlwifi: mvm: add registration to thermal zone")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>