mt76: fix possible NULL pointer dereferencing in mt76x2_ampdu_action()

Initialize mt76_txq pointer after ieee80211_txq pointer check.
Remove space after the pointer cast

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>