iwlwifi: mvm: move internally to use bigger INVALID_TXQ

We can't use IEEE80211_INVAL_HW_QUEUE to mark a queue as
invalid since 255 will be a valid value for a TVQM queue
index.
Use IWL_MVM_INVALID_QUEUE instead for accessing txq_id.
reserved_queue can stay a u8 since reserved_queue is not
used when TVQM is enabled.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>