iwlwifi: mvm: Fix tx-backoff when NIC exit thermal throttling

When NIC is exiting from thermal throttling state (i.e. after
heating and then cooling down), tx-backoff values are assigned
to be zero, instead of being restored to the minimal value. Fixed.

Signed-off-by: Eytan Lifshitz <eytan.lifshitz@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>