iwlwifi: tt: move ucode_loaded check under mutex

The ucode_loaded check should be under the mutex, since it can
otherwise change state after we looked at it and before we got
the mutex. Fix that.

Fixes: 5c89e7bc ("iwlwifi: mvm: add registration to cooling device")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>