mt76x2: fix possible NULL pointer dereferencing in mt76x2_tx()

Fix a theoretical NULL pointer dereferencing in mt76x2_tx routine that
can occurs for injected frames in a monitor vif since vif pointer could
be NULL for that interfaces

Fixes: 23405236

 ("mt76: fix transmission of encrypted mgmt frames")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>