iwlwifi: remove lock around txq_enable

This locking isn't needed. The only locking we need is when
we access prph registers but there is already a separate
lock for that.
Since we haven't returned from the mac80211's
IEEE80211_AMPDU_TX_OPERATIONAL ampdu_action, we cannot
receive any Tx frame for that sta / tid while enabling the
queue.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>