iwlwifi: mvm: handle iwl_mvm_tvqm_enable_txq() error return

iwl_mvm_tvqm_enable_txq() can return an error, notably if unable
to allocate memory for the queue. Handle this error throughout,
avoiding storing the invalid value into a u16 which later leads
to a disable of an invalid queue ("queue 65524 not used", where
65524 is just -ENOMEM in a u16).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>