mt76: fix possible pktid leak

Fix a possible idr pkt-id leak if the packet is dropped on tx side

Fixes: bd1e3e7b

 ("mt76: introduce packet_id idr")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/a560caffcc24452fb48af53904bbe5c45ea5db93.1637602268.git.lorenzo@kernel.org